-Are parents okay with you buying a bike? (Have to ask, since you're 13)
-What are you looking for, used or brand-new?
-What is your budget?
-Do you have any skills with tools? (Trust me, owning one of these, you WILL learn how to use tools!)
-What kind of bike are you looking for - minimoto (the smallest), midbike, superbike, mini-chopper, dirt bike ...?
You may want to check on your local craigslist - use "pocket" as the search, and search in "all for sale". Used bikes occasionally show up there ... Be forewarned: these are USED bikes. They will range anywhere from "just needs a little bit of work" to "bike is completely and thoroughly trashed, only usable for parts." (That's almost how that bike was when I got it. <-- ) The good thing about used bikes is you can usually get them for 1/3 to 1/2 the cost of a brand-new machine.