Shoulder Wall Pushup
This exercise is great to build up to a full push up. It allows you to build strength while focusing on your mechanics without infliction of pain.
Start: Position yourself near a wall. The closer your feet are to the wall, the easier the exercise.
Place your hands directly inline with your shoulders. Keep your elbows in and perform a push up. This will isolate your triceps. You can isolate into your pectoralis muscles buy widening your hands and let your elbows fall out.
You can progress by lowering your body to a countertop, then to the floor but your knees down.