Step 1: Do push ups daily. a good amount to do is about 30-50 per day.
Step 2: Frequently do sit ups. About 100 per day would be a good amount to stay fit.
Step 3: Run laps around your community and time yourself to keep your endurance up. You could also play sports like basketball for a good cardio workout.
Step4: Maintain a healthy diet by eating good foods. You should eat a lot of fruits.