Well, the squat is the king of weight-lifting exercises, so it is best to start there. But make sure that you do it in a safe way and have someone show you good form (I'm assuming you aren't an experienced lifter).
I'd advise sticking with the basic compound movements: squat, deadlift, bench press, rowing, pull-ups, dips, etc.
You should also clean up your diet and drop some weight for tennis. Djokovic is about your height and almost a 100lbs lighter. That definitely makes it easier to move around the court. Good luck.