Due to the hazardous nature of refrigerants, only specially trained personnel should perform AC maintenance tasks. However, if you have the proper training and skills and wish to undertake this type of project and save quite a bit of money in the process, you will need to purchase, at a minimum, a triple gauge set. These gauge sets can be purchased at most auto parts stores. The triple gauge set will tell you three things: How much pressure is present on the pressurized side of the system, how much pressure is present on the suction side, and how much pressure is in the supply or bottle.
For more info, see How to Recharge the Air Conditioner in Your Car