Have the system evacuated by a shop. Change the o-ring. Use lube on the new o-ring. Do not overtighten; use the shortest wrench you have so you don't get too much torque on there. Take the car back to the shop, have them pull a vacuum, then have them add the correct amount of refrigerant (there is a sticker on the car with the info).
Or just have them do everything. Changing the o-ring is minimal effort; the rest you'd pay for anyway.