You're too vague. Does it die? Does it get better when the engine warms up? Does it run okay at higher revs and only has a problem at low RPMs? Is your engine carburetted or fuel injected? I'm assuming the idle is too low and doesn't improve when the engine warms up but runs fine when it's in higher revs, in which case it could be your O2 sensor, EGR valve (if you even have one), fouled up spark plugs, bad plug wires, clogged air filter (easy to check, start there), worn down distributor points - if it's carburetted then maybe it's running rich or lean, and maybe the idle speed is set too low; if it's fuel injected then maybe your injectors are plugged up and you need to run some Techron or injector cleaner through your tank. Have all these things checked out - if it's not any of them it could be something more serious, but if the car runs fine at higher revs then it's probably a minor issue that wouldn't take much to correct.
