Is there a local all star gym near you? Your first step should be to contact them and see if they're looking for coaches. It may be as easy as that. If they're not hiring for the season, you could ask if they would be open to having you volunteer with some of their teams. Explain to the gym owner/manager that you'd like to coach all star in the future and would love to get some hands-on experience. Accept any position, paid or unpaid, tiny level 1 or senior level 5. Once you get in there, show that you can be hands on and helpful. Don't be afraid to jump right in-- coaches and gym owners will appreciate that you don't stand around and wait for an invitation. However, if you're volunteering, you'll need to be careful not to step on the coaches' toes. You may want to ask "may I give him/her some tips?" before critiquing athletes.
ETA: I also knew I wanted to coach when I moved home from college. The summer before my senior year, I applied to work at the all star gym as a tumbling and stunting coach. I was able to prove my coaching and teaching abilities and earned my coaching job when I returned home.
