Push-Ups
Chin-Ups
Squats
Crunches/Leg Raises
You can vary the hands/grip on both Chins and Push-Ups to target different parts of the muscle.
Ron Jones has a massive list of
Body Weight Push-Ups as well as a
bunch of other exercises.
Start with what you can do and just keep adding as you progress.
My boxing trainer made me tone down my weights and focus on lots of push-ups and they definitely work! My working sets now consist of something like 80 push-ups in 4 different positions ...