x2 on JL's comment. A/C systems involve high pressures and low temperatures that will cause serious equipment damage and personal injury. While I encourage people to learn how to do it, this is one area not to go off and wing it on your own.
Yes, there are safeguards, such as different sized high and low pressure fittings, along with color-coded port caps. But this is not an idiot-proof system.
Understanding how A/C systems work is vital to solving the problem - yes, the majority of times it will be a case of low freon, but there are many other reasons as to why the system is not performing as it should. Eye and hand protection are a must if one chooses to do-it-yourself.