Back wheel cog

Loosen the nut that holds the back wheel and pull it back until the chain is tight and then retighten the nut. If when the chain is tight, the wheel is not on the rear frame, you need to take a link out of the chain.

any bike shop or toys-r us- or toy world bikes.

If the tire on the rim is still holding air and the rim isn't obviously bent it should be alright if you go nice and slow but would still be a better idea to put a spare on it if you got it. Slamming off the road though it's possible you just threw it out of alignment which would be great, but then again you could have bent part of your steering linkage.

Put the chain to the inside of the front cog wheel, beside the pedals, first but not onto it, so you have plenty of room for pulling the chain to the rear. Fit the wheel back into place and before tightening it pull down the spring loaded arm, with the small cog wheel beside the back wheel. This takes the tension off the chain and allows you to put it into place on the back wheel. Tighten up the wheel. When it's tightened fit the chain onto the top of the front cog wheel. Lift the back wheel off the ground and turn the pedals forward. The chain will click onto the front cog wheel when you have turned the pedals one complete turn.

If you need to see a picture of how it should look at the back you can see one at http://orangecounty.craigslist.org/bik/2409696114.html

Put the chain on the gears, loosen the back wheel at the axle and pull it back - that's how you take up the slack.

There's a trick to putting a chain back on a bike. First fit it onto the back wheel. Then hold it on the top of the front cog wheel (at the pedals) Get someone to lift the back of the bike so the wheel is off the ground. While the wheel is off the ground and you're holding the chain on the top of the front cog wheel turn the pedals forward, with your other hand, as when you're cycling and as it turns the chain will go on.

If you don't have someone to hold the back wheel up off the ground you could hold the chain on the bottom of the front cog wheel and turn the pedal backwards. The chain will still go on.

With the wheel off remove the two pins which locate the guide wheels on the deraileur. Take off the guide wheels and the guide plate. Now straighten the chain and unwind the snarled area. Locate the chain around the smallest cog wheel on the back wheel and mount the wheel to the bike. The chain will hang slack. Now mount the upper guide wheel and guide plate to the deraileur making sure that the chain feeds off the rear cog and passes around the guide wheel towards the front of the bike. Push the deraileur forwards relative to the chain and slip the lower guide wheel in between the guide plates and pushing the chain back in the guides & fasten with the pin. As you reassemble the plates and wheels on the deraileur be sure to observe carefully which way the chain is running. It is quite logical and this should prevent mistakes being made

Pull the chain out slightly, away from the bicycle frame, at the bottom just behind the front cog wheel (attached to the pedals). While holding it out turn the pedals backwards and the chain will come off.

Wash, degrease and scrub the chain and derailleurs with a good degreaser (like Simple Green). Dry the chain and relubricate it with an appropriate bike chain lubricant (not oil or WD-40). Shift the derailleurs to the smallest chain ring and cog on the cassette (low front gear, high rear gear) and then loosen the cable at the derailleurs, pull the cables taut and reattach them. If the derailleurs still need adjustment, here is a procedure for it:
Flip the bike over (so you can pedal the bike manually while you shift) and shift the front to the middle chain ring. Shift the rear to the highest gear (small cog). Release the cable from the pinch bolt on the derailleur. Adjust the screw marked "H" on the derailleur until the jockey wheel on the derailleur is aligned perfectly over the small cassette cog. Set the cable adjuster on the derailleur to the middle of its range (find this by screwing it all the way in and then count how many revolutions it makes until it screws out completely. Screw it back in one half of the total revolutions). Reattach the cable and shift to the lowest gear (the biggest cog). Over-shift and hold on the shifter, then adjust the "L" screw on the derailleur until the jockey wheel sits just past the last cog. You have to hold the shifter to do this. Now shift into the middle gear (or one of the two middle gears if the bike has an even number of gears) and adjust the derailleur using the barrel adjuster you previously set to the middle of its range. Adjust it so that the derailleur is sitting perfectly over the appropriate gear. Shift up and down the cassette while pedaling and in every gear, reverse pedal (freewheel) to make sure the chain does not hop. Adjust as necessary until all gears are smooth.
Now the front:
Set the rear derailleur to the lowest gear (big cog) and the front to the lowest gear (small chain ring). Disconnect the derailleur cable. Screw in the adjuster for the cable tension all the way (this adjuster is on the shifter of a mountain bike or the down tube of a road bike). Pedal to make sure the chain is not rubbing on the derailleur cage. If it is, adjust the "L" setting on the derailleur until the chain moves by without touching the cage. Now pull taut and reattch the cable and shift the front to the big ring while pedaling. If it does not shift up well or completely, hold the shifter past its normal shift and adjust the "H" screw so that the derailleur allows the shift. Shift into the highest cassette gear and keep pedaling. The chain should not be touching the front derailleur's cage when you pedal. Adjust with the barrel adjuster until the chain moves freely without touching the derailleur cage.
That should be it, unless there is another mechanical problem causing the missed shifts, like a bent derailleur hanger or damaged teeth on the cassette cogs or a bad derailleur. If it is still problematic, come back for further information.

i hope this helps
-matthew

you need a new coaster in the back rim. theres little gears inside that grab when you pedal forward and not backward. you stripped/broke the gears inside. take the back wheel off and if the sproket/cog spins both ways then you need to have it replaced.