Alma, one of the great Book of Mormon missionaries, invited those who were not members of the Church to "come and be baptized unto repentance" (Alma 5:62). You can follow his example.
Be a good friend and example to friends and family members who aren't members of the Church.
Seek opportunities to teach your friends and neighbors about the gospel.
Learn the gospel and practice teaching it now. Ask your ward or branch mission leader for guidance on how to teach the gospel. If possible, attend lessons when the full-time missionaries teach investigators.
Study Preach My Gospel with your parents for family home evening. You could take turns talking about the lessons, teaching parts of them, and inviting each other to bear testimony of what you learn.
Serving a mission is an important goal to make now, and preparing now will benefit your life every day leading up to a mission. You're never too young to start preparing—you don't have to wait until you've grown a foot or two.