There are several "ways" to approach an A/C problem depending on how much you care about your car. Most people would say to spend some time diagnosing whats really wrong but most likely your A/C is not working because of a lack of freon. The best way is to roll into your local expert's shop and let them do it correctly and part with a lot of cash in the process.

The first thing you want to do is see if you can get the compressor to engage at all. Read through the owners manual and get familiar with the controls of the A/C system.

If your unable to get the compressor to come on its from two possible sources one being an electrical issue and the other being a lack of coolant issue. The electrical issues are less common and a bit of a pain to trace. More common are leaks in the A/C's systems.

The lack of coolant can be from a number of different ways. The "quck and dirty" way to approach this problem is to get one of the R-134 kits from the local autoparts store and do a partial conversion. If you get the kit with the guage on it (which I would highly reccomend) you can check the pressure of your system. Most likely its empty or way under pressure. Keep in mind that its illegal to release your R12 into the atmosphere for the conversion. Don't charge the system if you have correct pressure as it may not help and you will be destroying some possibly expensive R12 that you have in the system that cools better. At this point you may have identified you have a leak some where in the system. If you have a leak you can "try" to charge the system using the instructions in the charge kit. I would also reccomend a can of leak stop first to plug any small leaks in the system. 9 times out of 10 your A/C will start cooling and your compressor will come on. The A/C system is equipped to shut down the compressor under a low coolant condition to keep from destroying it (the coolant has the lubrication in it). At this point if your really really lucky your system will keep cooling and you wont have to do anything else. If your not lucky which many people are not you would at this point have to trace where the leak is coming from and replace the parts involved.

Before you dive into this portion of your adventure I suggest you develop some knowledge of A/C systems and how they function.

http://en.wikipedia.org/wiki/Air_conditioning

There does come a point where a DIY person should give up and move onto a professional. If you need to do a flush of your system or if you need to open the system for a part replacement (such as a new compressor) its best to let a professional evacuate flush and recharge the system. If you have a slow leak its unlikely air and other contaminates have made it in the system.

Wish you luck on your A/C work! Hopefully you will be lucky and just have a slow leak that can be fixed with leak stop.