Learn how to ride a bike can be a real challenge if you are not a child anymore. The fear of falling down may be so strong that your mind will trick you and not allow your learning.
You are going to need all the support you can get.
But sometimes it is better less help, and more self-confidence.
Step 1 In order to overcome the fear, you should ask some friends to backup you while you are learning, since they are a more frequently company than your parents right now.
Step 2 Borrow a bike from one of them, and then find a safe place to begin.
Step 3 Find the perfect balance may take a little while, but it will come.
Step 4 Make sure you know how to use both brakes before going in the direction of a tree.
Step 5 Be confident of yourself, and try do not be so afraid of falling; there are your friends to help you if it happens (after some good laughs).