To keep the deltoids (shoulders) from doing too much work, don’t allow the rear deltoids to come off the bench, especially the last few inches when pushing the weight up. They should remain in the same position flat on the bench throughout the movement.
If you are having trouble balancing the weight make sure that your arms remain relatively vertical throughout the left. The more vertical your arms are, the better your balance will be.
The more narrow the grip, the more you involve your triceps.
The wider the grip the more the outer area of the chest is worked.