ok first of all, put down your tools until you've read this entirely.
If you have already opened your AC system i hope you recovered the system. if not you've let a lot of refrigerant into the atmosphere.
Secondly, anytime you open an AC line you introduce moisture, and it is a good idea to replace the receiver/dryer/accumulator.
If you are going to DIY, read this, if not, take it to a shop.
After you install all your new parts you are going to need to vacuum the system down and remove all the air from the system.
You really need to have R-134 AC gauges and a digital scale so you can regulate the amount of refrigerant, as some systems will not operate correctly or cool at all, if they are overcharged.
When you switch from the vacuum machine to the manifold gauges, the schrader valve can open due to the high vacuum in the system, so switch them quickly. dont go eat a sandwich and come back to hook up the gauges.
Charge with the correct weight of R-134, i cant remember it exactly, and check your AC with a thermometer. it should cool to 33-43 deg. depending on the ambient temp.