I have some training in this, but I'm not an experienced pro or anything. First, if you need freon added, you have a leak. Usually the leak is either the hoses, or a faulty compressor seal. The correct way to repair it is to verify what's leaking and fix it before adding freon.
If you have a super-slow leak (holds freon for the whole season, etc.), and you just want to do a quickie repair, you can try adding freon (12 or 134, depending on year of car). The problem is, that the system won't work if there's too much freon, just as with too little. You really need the correct amount, which is met by removing all of the freon, then putting the system under a total vacuum, then letting the specified amount of freon into the system along with the proper quantity of new refrigeration oil.
Now with the correct methods said, you can try getting one of those cans of refrigerant at WalMart, and the adaptor to put it in, and see if that works. Start the car, and turn on the A/C, and make sure that the radiator fan is on. Follow the directions on the can and on the adaptor you buy, and hold the can upside down and let the contents suck into the system, and the compressor should start up as soon as there's enough freon to un-trip the low pressure safety sensor off (turning the power back on to the compressor).
Honestly my best advice is spend the $30 or so, and have a shop do a quick diagnostic on it first, and let them put the freon in. At least it will have closer to the correct pressure, even if the leak isn't fixed.
Charles