just start riding your bike to easy places that are flat. Dont try to ride your bike on rocks or slippery areas. Find easy places to ride your bike and if you really like it, you can advance and starting climbing hills. If you really like that, you can try diff't terrains and see. I would go with someone when you start to want to try different things. There are many bike routes that you can also do. There are signs all over the place and all kinds of trails. But being you are only 13, i would go with some friends . Just remember to wear a biking helmet even if its easy.