|I'll respond to this one because I am doing this right now. Haha
Most vehicles that came from the factory with R12 now have the R134a fitted to low side, as most switch to R134a. Low side is all that is "needed" to charge the system.
R134a connectors are different sizes for low and high pressure. Low is blue, high is red.
Correct way to do this if you have an R12 that has a R134a adapter on the low side:
Put R134a adapters on low and high. Pull a vacuum. If it does not hold a vacuum then start replacing o-rings. If o-rings don't do the trick then you will need to check for leaks with dye, etc....
Once system will hold a vacuum then you can charge the system with new refrigerant.
Approx 36oz of refrigerant is used in the system.
Also note: If you do not know your ac systems history and it is opperating poorly then I suggest replacing the dryer unit. They are VERY cheap to replace and can make a huge difference if the system is performing poorly. Link for $18 replacement.