Fixez la balle!
feek-SAY lah BAHL! Click below to hear this.
Keep your eye on the ball!
This ball is not in need of repair. Fix the ball would be Réparez la balle. Fixez, on the other hand, means attach firmly, which includes what you do with your eyes when you stare fixedly at something.
La balle, by the way, only refers to some kinds of balls: baseballs, golf balls, pingpong balls and the like. Inflatable balls like basketballs and soccer balls are des ballons (day bah-LAW).
So you can also say Fixez le ballon if that’s the sport you’re in. But if you are using the expression metaphorically, outside of the sports arena, say Fixez la balle.