15? Well first you must know that many federations have age limits --- I know that for the bikini and modelling shows I do I believe you must be 18 -- so find out first. There are also federations that have teen/junior categories that you may be able to do. As far as sponsors go, same thing sweetie. They may or may not be interested, only because you will be limited to shows, your age of consent (for using pics etc -- remember this is about the physique, so pics showcase this and I can tell you what I would think if my 15 year old posed in a bikini let alone the legalities behind it). That howver is for a national type sponsor.
By all means, start local, I did. Get your community on your side and you may be able to work out for free (again it may be an age thing at your gym), get your suits done, tanning (try to limit this IMO), supplements (again, I hate to see someone so young rely too much on supplementation, go for natural foods where possible) etc. You may also be able to get some media in your community paper or even your school. Start locally, a
Also, from a muscle perspective, and I am no expert, but you have some time yet to build, I am always hearing/reading how there is a prime time for muscle maturaity...someone else on the board will be able to help you with this more.
Honest, for now, focus on the junior ranks, get your experience on stage, and by the time you are an adult (I know seems so far lol) you will be a pro and very little will stand in your way. This can be a sport filled with harsh lessons so take advantage of your youth sweetie and use your energy and genetics to start to build an awesome physique.
Must be becuase I am a mom