The two bolts that holds the toilet bowl down. one on each side. the whole things turn ad i cannot take it off

Put towel on floor, look underneath unit with a flashlight - look for a hole below the seat hinge area - screw inside turn screw (leftie- looses) Could be slot, Phillips head or other, like a hex head. Whatever - loosen both sides and check for slack by pulling up on seat, then remove screws down and seat up- done

Here's two things:
Look at the lid, with the seat down. The bolts are sometimes covered with a snap-in plastic cap or flap. If so, pry up with a screwdriver. Then use a screwdriver and wrench to tighten.
If the bolts appear to be cast as a part of the hinge assembly, you should be able to reach around the seat to the back side (no pun intended) of the toilet, and tighten with a wrench.
I sometimes use a 'mirror trick' to get a better idea of what's back there. Just use a small mirror and hold it to where you can see the bolt and not on the back.

There has to be either a nut and washer under the toilet, or if its reversed a bolt head with a washer under it holding it down. To remove it you can either use a hacksaw blade between the bowl and the bottom of the seat. Or use a dremel tool to slowly cut away the metal rod, etc., from the top. If using hacksaw put down some masking tape on the bowl so you don't scar up the enamel surfacr of the bowl. Work slowly and you will eventulaly cut through it.

Turn water supply line of to toilet. Flush and hold handle down to drain water from tank. Take lid of tank off and use a towel to soak up the rest of the water inside tank. Remove nuts on bottom of Tank bolts. Lift off and lay down on its side. Remove the rubber or sponge type gasket that is on bottom of the tank that covers the nut from standpipe assembly. Buy new gasket and install back on bottom of tank over Nut for standpipe. Put tank back on toilet bowl. Install bolts and tighten. reconnect water line and turn on.

The wax ring has gone bad. If the toilet moves when you set on it you might try snugging down the bowl bolts on both sides but be very careful not to go to tight you will brake the bowl.
If you decide to try to fix this your self with you buy a new wax ring buy the one with the horn in it they seal better.

Let's assume you can buy the tank.
Installation of tank is simple.

If you can buy the eljer tank, you can buy whole new toilet with bowl and tank from home center.

We'll walk through instructions for both options.

1) How to replace tank on existing bowl.
-Tank is attached from underneath with two nuts.
-Turn off water, and hold flush valve down to drain tank
-Water connection is one nut. Follow supply line up to where it connects to tank. Loosen the nut that is around supply line just underneath tank. Pull nut downward and notice a black rubber washer that is probably stuck inside nut, or is stuck at bottom of tank. Make a note of where that nut is because it seals the water line to tank.
-You might also have a flex connector instead of supply tube. In that case, just unscrew flex connect from bottom of flush valve.
-Look beneath the tank and there are two nuts that hold tank to bowl.
-Take off these two nuts. A ratchet and socket is best tool, but you can use crescent wrench. -Notice these nuts are not exceptionally tight, but they are firm.
-Straddle the bowl and hold tank with two hands and lift up. Tank will come right up.
-Notice that on bottom of the tank: there are 2 bolts sticking out and there is a spongy rubber washer. This is the spud washer. Your new tank will have new washer. This washer seals tank to bowl so water doesn't leak when you flush.
-Notice round opening where flush water flows into bowl. Spud washer seals this hole. If you have experienced slow flushing, or if you have hard water, pour several cupfuls of CLR Calcium Lime Rust into the hole. CLR is sold at Home Depot by the gallon for $10.

-Open box to new tank. Find spud washer and fix it to bottom of new tank.
-Now a new tank is slippery, I want you to set boxes on floor nearby so if tank starts slipping, aim toward a box instead of hard tile floor.
-New tank comes with 2 new bolts, with nuts. Look at instruction sheet and see exactly how they want you to connect bolts to tank. Look carefully where they want you to put washers. These washer seal the tank when you tighten bolts.
-Now your spud washer is in place, the bolts are attached to tank and are sticking out of bottom of tank.
-Straddle bowl and lower new tank into place. Make sure spud washer lines up. Bolts line up. And flush valve lines up with supply tube. It's that easy.
-Tighten the nuts. Remember, not too tight. Toilet is not steel. And you can tighten more if there is seeping water.
-Now tank is in place. Raise nut and washer that are on supply tube and tighten nut onto bottom of flush valve. This nut is fairly tight. You might want to buy a new washer.
-If you have a flex line for the water supply, then just connect the flex line.

-Turn water ON a little bit at a time to avoid gusher if something is not correct.
-Use paper towels to check for dripping or seepage beneath tank. Tighten nuts appropriately to end seepage.
-Flush toilet and look for seepage around spud washer.
-You're done.

2) How to install new toilet.
-Remove old tank as shown above.
-Bowl is held to floor with two nuts.
-Ideal world those two nuts come right off and you lift bowl up off the floor.
-Real world, those nuts are rusted and you have to drill through side of the nut and bolt to break it off.
-Lift bowl up.
-Have cardboard ready next to toilet so you can set bowl on something. Or set it in bathtub.
-Bowl will be damp and wax ring is on bottom of bowl and will mess up the floor.
-Notice how the toilet bolts are set in the floor flange. Take photo if needed so you can put it back. It's real simple, but if you have never done this, a photo might help.
-Take putty knife and scrape wax ring off of floor flange. You want it smooth, but you don't clean it.
-New toilet comes with wax ring and bolts, but you might have to buy these at hardware store. Get wax right with gasket. The boxes are marked. Buy two of them.
-Set wax ring in center of floor flange. Gasket goes downward. Rounded side of wax ring goes up.
-Set new toilet bolts into floor flange. Keep bolts straight up using wax from the old wax ring, or from 2nd wax right you purchased.
-Keep bolts straight across from each other.
-Time to set the toilet.
-Stand in front of bowl and grab on either side at point where seat connects.
-Here's the fun part.
-Bowl has to go straight down onto wax ring so you have to be strong enough to hold tank level while aligning tank with bolts, without knocking the bolts down. Like a carnival game.
-Straddle walk toilet while holding it up high enough so it will not knock over the bolts.
-Lower bowl into place going straight down using bolts as guide. Lower a little bit and check one side for alignment and then check other side until bowl is going down and bolts fit through the holes.
-If bolt falls over or gets pushed away, straddle walk toilet back and set bolt straight up again.
-Once bowl is aligned, keep pushing straight down on bowl, and finally use your weight to push bowl to floor. Be careful to keep bowl perpendicular to wall so tank will sit straight.
-Attach bowl to floor with nuts and washers.
-Install tank as shown above.

Add a comment for more free help.
Also take advantage of fixya phone service.
For a price, fixya expert speaks with you over phone while you work on toilet or any do-it-yourself project.

Leaks at the floor or the tank where it meets the base behind the seat?

Generally tight enough to sinch it so as not to leak.

If the whole unit moves on the floor the wax ring may need to be replaced. If the tank leaks be sure the bolts are tightened evenly with new gaskets sealing them inside the tank. Tighten the nuts evenly until it stops moving freely and the leak should stop.

The Gerber Ultra Flush Back Outlet toilet, that uses 1.1 Gallons of water to flush?

Just like any common toilet. Install the tank to the bowl of the toilet, set the toilet, install the flush mechanism in the tank, attach the supply line.

1.MAKE SURE you put the two plastic U-shaped spacers on the back of the bowl!
2.Install the round rubber gasket in-between the tank and the bowl.
3.Set the tank down on the bowl. Make sure you have the round gaskets that go up under the bolt heads, then insert the two bolts into the tank, and through the bowl. NOT installing these will cause a leak!

Tighten these bolts down, until the nuts are JUST touching the tank. (There is a flat washer that goes under each nut)
DO NOT tighten one bolt all the way, then tighten the other bolt!!
These bolts MUST be tightened evenly. Tighten one turn on one bolt, then go to the other bolt.
DO NOT overtighten, or you will break the bowl, and/or the tank!

Make sure your toilet flange on the sewer pipe is in good repair, and clean, from the old beeswax ring. It can be messy, even using gloves. Use a flat painters stick to help remove the old wax ring. Scrape off into a old can. The wax thin residue on the floor and flange is fine.
Set the new wax ring in place, remove the plastic liner that comes with the ring. DO NOT leave that thin plastic liner! It's just to install that wax ring!

Install the bolts that hold the bowl to the floor, or reuse the old bolts if they are in good repair.
Tighten EVENLY, and don't overtighten! You can crack the bowl.

Now install the toilet mechanism in the tank.
Here is a 1 Page, Tank to Bowl Installation Guide from Gerber. It is a PDF file. Let it download ALL the way before looking at it.
http://gerberonline.com/fileLib/Toilets%20Ultra%20Flush%20IS.pdf

Should this direct link not work for you, here is the page it came from,
http://www.gerberonline.com/Catalog.aspx?categoryID=2204&itemID=3514

It's on the right side -> Installation Instruction (PDF)

is the black thing between the the tank and the bowl?
if so, go to any of the home center or plumping store for the seal.
shunt off the water and unbolt the 3 bolts that holds the tank to the bowl from the buttom. you may need a flat screw driver to help hold the bolts. pull up the tank slowly, and lay it on the side, replace the seal, reseat the tank on to the bowl and tighten the bolts. make sure all bolts are equal pressure and the tank is level.