what are the most common causes of A/C problems?

Probably the biggest single failure in air conditioning units is loss of refrigerant charge.

After that, there is a long line of defects which can cause a failure to cool properly.

If you dont know what you are doing, you are probably well advised to take the car to a competent mechanic.

If you want to buy a charge kit with a simple pressure gauge, you might be able to "top up" your refrigerant charge, and it might return the car to normal cooling, for a while.

A kit like this is probably less than $25. It is a simple task, but if you dont know what you are doing, you can spend your money and still not get a fix. At worst case, you can hurt yourself.