First, you start by finding a trail and riding your bike on it. look for tricky sections with rocks and short hills, pratice them until you can ride without stepping off the pedals.
Second, ride whenever you can; to and from school, friends houses, errands to the store for mom (bring a backpack for the goods). Go on long weekend rides,
For race listings, try:
www.bikereg.com
www.usacycling.org
If you're lucky and live in an area with a highly active racing community, there will be many other resources to choose from. You should always start in the beginner or novice category. If you open a can of whoopass, then you can upgrade.
Go to a local bike shop and ask about clubs and group rides.
You also might want to consider a road bike, or road tires for your mtb. Road cycling is an essential component to endurance off road.
Track and field are good for overall fitness but not so much for cycling specifically. The muscle groups are used so differently that extended training in one discipline is detrimental to the other. Cyling is way better to build fitness and endurance for running and track events than the other way around. This months issue of Runners World has an excellent article on bicycle training regimens for competitive runners.
"RIde Lots" - Eddy Merckx