Death2000 is right on the mark, as usual, except in his choice of vocabulary. He meant oxygen displacing rather than oxygen depleting (damn spell checkers).
134a refrigerant is the same stuff they used to put in 'canned air' that you use to clean the dust out of electronics. Modern automotive A/C holds only a couple of pounds of refrigerant, and when you lose a half a pound (normal loss for 11 years of service) it starts blowing warm. You could have any of several other problems, but on a car this age, the smart money is betting on it being just a little low on refrigerant.
If you are not familiar with A/C, working with high pressure gas around a running engine is not a good idea.
Any shop that does A/C work should be able to handle this for you for $50 or so. Just don't let them point out the chattering noise from a worn belt tensioner and try to sell you a new compressor.
If you decide to get a book and buy some refrigerant and try it yourself,
1) be absolutely sure you know which connection is the low pressure side.
2) be careful not to overcharge it. Too much refrigerant is worse than too little.
3) don't buy the refrigerant with leak sealant in it. The A/C shop that works on it later will hate it when you gum up their expensive refrigerant recovery equipment with stop leak.