To recharge your air conditioning system you'll need pressurized refrigerant (sometimes referred to as freon) and a pressure gauge to keep track of how much is in the system. There are lots of different AC recharge tools you can buy, but most are for the professional and are pretty expensive. If you have a fleet of vehicles you plan to maintain, go ahead and spring for your the real deal, but if you're air conditioning maintenance is limited to a couple of family cars, I suggest one of the all-in-one kits. These kits consist of a can of R134 and a pressure gauge all in one. They work well and are very easy to understand, even for somebody who has no experience with AC.
The kit we chose had the pressure gauge clearly visible in the cap, so you know what you're getting. Any all-in-one kit is probably fine.
You might also want to check for leaks in your system to be sure it's healthy.