First make sure you wear a helmet. Now theirs a lot of videos and websites on how to ride a bike likehttp://www.wikihow.com/Ride-a-Bicycle www.youtube.com/watch?v=gjUopYKMVhA www.youtube.com/watch?v=sPfR_VjVcZ8
Now have someone hold the bike so you can get on and learn the feel of sitting of the bike, it will be ruff at first but just keep pedaling and if you feel yourself fall you have your feet to catch your self but if you do fall just get back up and try again. It's a skill you have to learn so it takes practice, but I know you can do it and I hope you have a life long bike riding hobby.