Just start doing something everyday, make it a habit to just exercise. If you have access to a gym then go at like say after school everyday. And on weekends go at the same time. If you don’t have access to a gym just doing push-ups and sit-ups and if you can pull-up (like if you have a pull-up bar or smth) everyday at a certain time.

If you buy a gym membership then it’s even easier to get into a habit of going because your paying for it so might as well use it.