How can I start cross-country mountain biking?

I'm 16 years old and I looove mountain biking. I do track, so I have a lot of endurance and I have good muscles from gymnastics, which will help me a lot with my biking. I bike a lot but I just do it for fun– I'd really like to compete in a race, but I've never done a biking race before. How can I start? Where can I find a local race? How do I figure out what ability level I should enter in? I know track helps a lot with training for biking, but what else can I do? We also weight train a lot–which is good because I don't do gymnastics anymore. Any answers are greatly appreciated :)

Go to a local shop and inquire or research state MTB clubs. There is also IMBA or NORBA the govening bodies of MTB racing. Look them up on the web.