prepare to grow about 8 years older. Prepare to be rejected when you try to join. You're 13.
O.k. on a serious note, don't get into trouble. At least with the law. If you know anyone now that you plan on still knowing when you join, don't piss em off and try keeping them on your good side. I've known some that have failed investigations when their BI checked references and got bad word from some disgruntled people. When you're in high school try taking electives that may help like business. Not sure if there's criminal justice but if there is take it. When you get out of high school, I'd work on getting a degree ASAP while you're young. In hindsight it's something I wish I'd done. I'm now 29 and barely working on mine. Time is precious.Try looking into the Explorer Program and see how old you have to be to start. I'm sure there is a lot more you can do to prepare but I'll leave it up to LEO's to tell you. That's all I got for now.

You have one more year before you can join explorers. You have to be 14 AND in high school. Also the most obvious things you can do is to get good grades.