What could be wrong???

I have a pontiac Grand Prix SE 1994 I can start it but it dies right away I restart it it keeps dieing unless I give it just a bit of gas to keep the tac at 1.. When I pull up to traffic lights or stop signs it wants to die unless I keep the tac about 1-1/2. When she does run it is smooth sailing while driving, a bit of loss power when going up hills but all in all she does good for as old as she is.she will ldile once she is good and warmed up but the tac flucuates between 1 and 2. Ocassionally when I go to pull out in traffic she wants to die the tac drops out and I got to play with the gas to get her going out of the way of on coming traffic ( kinda scary at times) I dont know much about cars at all and I bought this car for only 500 because it was within my budget and I needed it to get back and forth to work. Not sure if it's a 93 or a 94 it says 01/94 inside the door but all the 94's I see have regular front ends this has lights all the way across the front. would really like to keep this car running because she seems to get good gas miliage.... can someone please help me to figure out what is wrong with it  why it wont run right for me??? Is this something that will be easy to fix and fairly cheap or is it something that will cost me and arm and a leg and I would be better of getting rid of it?  Thank you for being here with out you all I would be totally lost !