Unfortunately, your system runs on R12, the "old stuff". You could still get it recharged with R12 at a shop, as long as your system is in good shape with no leaks. R12 is just kind of expensive, but not as expensive as a conversion. You must go to a shop to have to charged with R12 because you cannot legally obtain it unless you are licensed by the EPA.

A conversion is a big deal. You cannot just empty and recharge the system - you'll destroy it very fast due to the incompatibility of old stuff and new stuff (R134a). To convert, you have to evacuate the system using an electric vacuum pump and replace the receiver-dryer at a minimum. If there is a way to drain the old oil out of the compressor, you'll want to do that too. You'll need to buy a manifold gauge set for around $50-100. Before you do anything, you have to make sure that there are no leaks in the system (and it probably has some, since that is why A/C's ultimately stop blowing cold - all the refrigerant leaks out).

A/C systems are kind of temperamental, require special tools, and are easily messed up by the novice (don't ask me how I know), and unless you are willing to put in the time to learn how they work, and how to find and fix leaks, you are better off taking the car to a mechanic. If want you teach yourself how to work on your A/C, I recommend the Haynes manual on automotive air conditioning.

-John