I had a new water pump and timing belt put on my 1991 lexus ls400, but i see white smoke coming from the tailpipe and when i put coolant in the reserve tank it leaks out but not on the ground. Can someone tell me what the problem is?

Probably coolant leaking into engine block through head gasket.
If it stops when warmed up, could be a sign of age but if keeps blowing white smoke, check your coolant levels to see if they are dropping above normal.

White smoke coming out of the exhaust on any gas powered vehicle is a sign that you have a blown head gasket allowing water from the cooling system to enter the cylinder, and when the piston comes up for combustion stroke and fires it turns the water to steam. This is also bad for allowing coolant to get in your oil, which anti-freeze in your oil will ruin the journals (Bearings) on the crankshaft (Major Engine Repair) costing several thousand dollars *NEW* !

Check the oil dip stick if it is a milky white this is really bad being cracked engine head or blown head gasket.
More than likely and best out come it will be the radiator or a radiator hose.
If you can see water stain some where around the radiator it does not need to be a big amount.
Or a radiator hose that may have water around it or coming out when hot then that is your leak.
Maybe a good idea to take to a radiator place or a mechanic ask them to put a pressure test on the radiator.
Also only fill radiator when cold or if you need to do when hot make shore the car is running to avoid cracking the engine head as this will make any grown person cry $$$$
And do on a hill with engine facing up running and cap off little bubbles will come out keep filling when bubbles stop put cap on fill reservoir ( a little container next to the radiator).
Let me know if this helps.
Regards Craig

Do you have to ad coolant very offen? White smoke may be an indication of engine coolant getting into the compression chamber.Sometimes you can remove the oil filler cap and look at the underside of it for a white milky residue
if its there you should have a shop look at it and take a compression test.Hope this helps.

you can go to this link it is a detail of the whole process it also has pictures
http://www.lexls.com/tutorials/engine/timingbelt.html

are you sure you didn't blow a head gasket?is it whit smoke?

Whatever you do, do NOT continue driving it this way until it is fixed. Hydraulic lockup can bend a piston rod ot even detonate the engne.
I think it got so hot at one point that it blew a head gasket.
Have a qualified mechanic perform a cooling system Pressure check.
If it holds pressure, which I doubt, it may be antifreeze running into your cylinders from he intake manifold. But the pressure test should tell you which head gasket it is, but have BOTH replaced, since they both underwent the stress.AND while replacing those gaskets, a new intake gasket will be installed too, as a matter of course. The price is about the same, since they have the top of the engine apart anyway. Just for information: White "smoke" is vaporized coolant or water being pushed out the tailpipe from the engine. Blue "Smoke" is oil, and Black "smoke" is soot- too much unburned fuel.

the engine is leaking coolant internally, via either a bad head gasket, cracked cylinder head, or cracked block.

Smoke is a sign of engine burning oil, which is caused by faulty piston rings, in most cases it's cheaper to have another engine installed in the car, instead of having work done on pistons. Hope this helps and please rate the answer if it was helpful, it helps me, thanks! P.S. Comment if you need more advice, it'll be my pleasure to assist you.

you need a new alternator