I started coaching my cousin and her friends in my back yard when I was 14 to help them get ready for tryouts. That turned in to me coaching their Jr high team while I was a junior and then the JV team the year I graduated. 19 years later I'm still coaching. I think starting young is the best way to go because it takes many many years to gain the experience you need to be truly successful.
Start by helping friends, cousins, neighbors at their homes. When even those "helpless" kids start making the team, parents will blow up your phone wanting you to help their kid too.
If you want to get paid, go to your local gym, YMCA, rec league--whatever is available and apply OR set up "house call" privates for a fee. Private lessons will make you some BANK if you do a good job. Those are even better because you set your own fees and hours.