To add some stuff to what Bonesaw has said. Try not to be in the gym for more than an hour at a time. Your muscles will only be primed for work and growth for so long, an hour time limit is a pretty good basic rule.
For me an hour normally means ~6-8 exercises depending on what they are. Im not gunna tell you exactly what exercises to do, but keep it simple and straight to the point. Check out the online journals to see some exercises people do. I have one one titled "Sawyer's Journal" that has some workouts I did a few years ago that are geared towards adding muscle and bulking up if that is your goal. Eat good and eat alot too.
EDIT:
Also, you say youre only 15, depending on far along you are from a growth/puberty stand point you may want to change your workouts. I might avoid really heavy weight and compound lifts if you are still growing at a fast rate (could stunt growth). Others would know more about this than me though.