I would do something like this:
- Every Tick
- If Ball.Y > AIPaddle.Y
- > Set AIPaddle.Y to AIPaddle.Y +2
- Every Tick
- If Ball.Y < AIPaddle.Y
- > Set AIPaddle.Y to AIPaddle.Y -2
The first one would cause the AIPaddle to move up if the ball is above it, the socond part would do the opposite.
For different difficulties, you can change how far it moves for different speeds.