I have water in my liftgate. How do I drain this water and how do I prevent the problem from recurring.

Check the drain holes under the lower gate. There are holes their for drainage.

First is to identfy the source of entry, using a hose. It could be from the high mount lamp area, or an unsealed body seam. Once identified, repairs can be made using seam sealer, urethane, or whatever it takes to stop the water entry, and maintain appearance.