Like any new endeavour I suggest starting in small steps and getting help when and where you can. Go to a bike park or join a bike club and get with some people who are willing to help you and show you the ropes. Rent a bike at first or buy a cheap mountain bike so if you bang it up or decide it's not for you than you haven't wasted a lot of money. Good Bike parks do charge but you have everything right there - rentals; groomed trails and guidance. Take a friend - go on a beautiful day and stick to the beginner trails - you'll be hooked!