*Volunteer at a racetrack. Start out mucking stalls and taking care of the horses. Talk to people. Talk to trainers and tell them you're interested in apprenticing. You can hot walk, groom, etc.
*As an apprentice jockey, you are expected to weigh about 115-120. This is with the 10lb weight allowance of being an apprentice.
*You must be at least 16.
*I'd start watching calories and making sure you do work out. Pilates, yoga, swimming, cardio, etc. are all good things to focus on.
*The biggest thing is to talk to people. Get yourself known around a training barn and be there at the crack of dawn. Once you have established that you know your stuff and are willing to put in hard work, talk to them about breezing a horse. You're going to need a lot of connections. It's all about who you know.
Each state has different requirements for getting your lisence when you actually do start running.
You want to learn the ins and outs of the backside and training before you start running. The more prepared you are the first time you get on the horse, the more likely you are to have people let you ride in the future.
Understand betting, nutrition, training, etc. You need to be able to communicate with the trainers and owners and follow out what they want.
Like someone said, make some phone calls. You're not going to start out working at Santa Anita, but there should be plenty of smaller barns nearby running smaller races that need an extra pair of hands. Tell them you want to learn and be honest -- Then work your butt off.