You'll likely have to have it converted over to the new refrigerant since the old stuff is almost impossible to find or is super expensive.

Now is probably a good time of the year to get it worked on, since few people are thinking about it and prices for parts and labor might be cheaper.

If you know nothing about AC, find a good local shop and have them recharge and test it. Assuming it's out of refrigerent, the old stuff is probably too expensive to test with, so they will probably want to convert over to the new stuff, which involves the replacement of some seals and a few other things.