Posting there will get you a lot better help than here.
I'm sure KC can help you more with this. What are you looking to do gain weight, gain muscle? Or just a start on trying to eat well?
You can start with eating a breakfast and 5-6 small meals a day. Sometimes it's hard, I know I'm 15 too, to be in control of what you eat in your house. Try getting your parent/guardian to buy whole wheat bread, that's a start. Staying away from fast food and restaurants will help too.
How healthy do you think you are now? What do you eat on a daily basis?