Do straight leg lifts, hanging if you have access to a bar or lying down (make sure the small of your back stays on the ground) if you don’t. If you can do them with your legs straight start with bent knees and straighten them as you improved. Build up to 3 sets of 20 reps three times a week and you’ll have strong abs.
If you want to add to that then do planks and side planks as well. One minute in a regular plank followed by one minute on each side is a good start and if you have it in you go back to the regular plank and hold to failure.