The pushup. One of the most widely known and used exercise. Even for weightlifting gym rats.
The pushup is an excellent exercise to have in any calisthenics program. There are so may variations of the exercise that you can keep progressing to newer ways to do them.
Assume a straight arm plan position. Make sure your back is straight like a board, do not hunch. Hands are to be shoulder width apart. Drop your body as low as you can. If you have the strength go all the way down. If you do not, try to at least go 90 degrees. Once you hit that spot push back up.