Simply putting one can of freon in is probably not enough to make a noticable difference in the air temp. This vehicle probably handles around 48 oz. of freon if it's empty.
I agree with 12oz in that you really must remove the air/water that is in your system. If I might offer you a suggestion, pick up a vacuum pump and pull a vacuum on it. There are two ways to do this. Autozone does rent them for free (although you'll be out about $200 until you return it), or you can go to harbor freight(if one's nearby) and pick up a vacuum pump that runs off of an air compressor. I paid around $15 for mine and it'll pull a vacuum to 28.3 inches of vaccuum. I swear by that piece of Tiawan crap.
Once you've got the vacuum, you can insert the freon into the system (+plus some oil). I suspect you'll find a small leak. Hopefully it's only a small leak and you can put in a can every couple of months to get you by. Also, when you charge it, add a shot of UV dye to the freon charge. It's available in the auto parts store and will help you locate any possible leaks.
All of this is dependent on the fact of you getting an a/c gauge. Trust me, they're pretty expensive if you don't have one. You might check to see if you can borrow one from the parts store (mine doesn't have them) or a friend. If that doesn't work, I'd buy a cheap set off of ebay or craigslist. Hopefully you can make it work. I couldn't drive one without A/C.
Mark