Welcome to the site, Mat. Let me say that it is refreshing to see a 15 yo with such good online manners (using please and thank you - I'm impressed).
I haven't been 15 for quite a while (I'm 50), but I can give a little advice. First, don't skip breakfast. It's important to start your day with good nutrition, and proper foods that can convert into energy (and muscle mass). Protein is essential to repair (build) muscles that are strained during weight training. So if you're not getting enough protein through the normal foods that you eat, a protein shake supplement right after a workout helps.
Like all things, moderation and balance are the keys. A healthy diet includes variety (carbs, protein and healthy fats), without going to extremes (too much or too little of any item). At 15, you still can gain significant height, size and mass, so just try to eat as healthy as you can.
That's just a start - good luck on your journey.