When you are writing an autobiography as a class assignment, you do not have enough time and space to include every aspect of your life to date. This means that you are probably best off choosing one main idea and purpose to focus on, and the idea and purpose should be stated in the introduction. What events and circumstances in your life have made you the person you are today? These are called "formative" events and circumstances. If your main idea is that some events and circumstances have made you who you are, then your exploration of these will support your main idea and give your essay purpose.
In addition to stating your main idea and purpose, it is always good to give the reader a little "preview" of the topics you will cover. For example, if your formative experiences have made you an outgoing and athletic person, the topics you will want to develop are the experiences that made you this way, and those topics should be mentioned in the introduction.
If you begin with a statement about your personality now, share your purpose of exploration of your "roots" as this person, and offer the reader a preview of how you will develop and support your main idea, you will have a introduction!