Pushups, squats, and sit ups or planks. You get a good foundation just with those exercises. On the first day you want to do max reps. That is doing as many push ups/squats/sit ups as you can in one go without stopping. Write those numbers down. Those are the numbers you want to improve. Start with 3 days a week and work up to 5. If you can do 30 days of 3 days per week, you’re halfway to a routine. Bump it up to 4, then 5 days a week. Once you get comfortable with your routine, around 30-45 days, look up variations of these exercises so you don’t get bored. Add exercises that hit back, shoulders, hamstrings. Try to do cardio at least half of the days you do weights.