Your autobiography is the story of your life. Lots of times we consider our lives to be boring and think that nothing exciting ever happens to us. This is rarely true if we stop to think for a while. Your life is unique to you and you are being asked to describe it to others in your class or to your teacher.
Try asking other people what they consider to be the best ways to describe you, or ask people who know your life what they feel are the most important things that have happened to you. You may have just forgotten many items that should be included about yourself. Try making lists of things in different ways: Chronologically; by importance; by how much fun you had; etc. If you think of things in different ways you may trigger some memories that you are forgetting.
If you honestly cannot come up with any other information about yourself to include in the writing, try some "colorful elaboration". For example, a sentence like "I live in a small house", can be changed into something like "For as long as I can remember, I have lived in a small white house in a friendly neighborhood, with my family."
I've found that the best way to learn to write things that aren't boring is to read. Find things that are not boring to you when you read them and try to write in the same style as what you've read.