I'm also only 15, and yeah, I expierence easily adjatated (spelled wrong) back pain. Mine though is directly related to a car accident (happened while actually going out riding) last year. Always stretch and try to go through a full range of motion. Don't forget little warm up exercises either. Take it easy at first until to feel comfortable. Your whole day of riding can depend on if you build towards your speed, instead of coming out and trying to triple the first thing you see.
As AS and Skoot (I believe it was) said. Standing up is a big deal. I have learned to stand and stand more as I ride. Right now I'm gong through some handling issues with my bike, so due to instability the standing part is kinda difficult at times.
Good luck bro! Ride safe