Here is a list of basic exercises you should be doing.
Try to do them all and try to do them several times a day.
Intensity and frequency depends on what your current condition is,
so try to stick to these rules:
If you can’t do any more repetitions, take rest and strech.
If you are having swollen muscles, strech more and drink lots of water.
If you did not sweat, you were not doing it hard enough.

Try first these exercises:
Still push up – First try only to hold a position of the push up, so your body will adapt to muscular stress if it’s not used to. Keep your abdominal muscles contracted – keep belly in. You should be straight like a board. No faking.

Delayed push ups – it’s a combined exercise where you do the push up as slow as possible. This way you will be burning calories and keeping your body straight, so your abdominal muscles will work hard. Count to 5 for each phase of push up and on the bottom count to 3, on top to 3. Do 5 push ups, then rest for 30sec. Repeat 5 times.

Delayed push ups with one leg – keeping one leg in the air, perform the same as normal delayed push ups.