The only way to know your family history is to do the work yourself. Everyone’s family tree is different. Even siblings will have different family trees if they have been married and/or had children. Start with yourself and work one generation back at a time. Gather up your records such as birth certificate, newspaper clippings, etc. Keep a journal. Obtain copies of your parents’ birth certificates, their marriage certificate, etc.
You can get a free trial membership to Ancestry.com. There are also FamilySearch.org and Family History Centers operated by The Church of Jesus Christ of Latter-day Saints (aka the Mormons) where you can obtain assistance in learning about your family history. No, they will not try to convert you. Plus, at 15, you are still legally considered a minor and would need parental permission for the LDS missionaries to contact you.