I would honestly spend very little time on the internet trying to read about the gym, because you will confuse yourself. You're set with the program @dchris linked above, so you can just start lifting. Ask real life, older folks in the gym to help you with your form. Lifting isn't the most important thing right now anyway (see below).
Eat healthy foods - protein, carbs, fruits, veggies; you know what healthy is so don't get crazy. Don't worry about your actual weight, and don't stress out; you're only 13.
Most importantly (at least in my opinion) is to play a sport. Preferably several sports. This used to be commonplace, but doesn't seem to be so anymore. The more varied activities you do, the more athletic you'll automatically become. Plus it's fun, it's social and it will help you in life no matter what you decide to do. I've seen other folks post about this, but I really think it's overlooked. It will be scary to try out for something at first, but that helps make it worthwhile.
1. Play sport(s)
2. Eat healthy
3. Lift in real life (not on the internet)
Have fun - you're young!