How do you charge ac?

Get a charging kit from an auto parts store. This should include a gauge (or gauges) and freon. Your '98 uses the new type of freon so you don't have to worry about conversion. Follow the instructions on the kit. You will need to locate the charging ports (high and low side) and put the proper amount of freon in the correct port until you reach the right pressure. You'll need the engine running and the A/C on high while you do this. Don't overcharge - you need to use a gauge.