I highly recommend the intro to MTB class tought by Wes and Marty. It is given once a month on the first Sunday of every month. They just had one yesterday so you need to sign up for the next one right away. They get full fast. You will not only learn about our trails bu tyou will also learn things about biking equipment, riding skills, and actually go out on the trail and apply it. Read more abobut it in the main SWAMP web site.
In the mean time, if you don't have a bike, you will definately need to start visiting bike shops and test several of them. AJ's and I am sure others, may have test bike that you can actually take out to a trail for a day or so. Or you can just rent one for a week. I wish I had done this before I bought my first bike. Good luck.