How to best get in shape for the summer?

Hi. I'm 16 years old, male, 165 pounds, and 5 foot 7. Yes, I'm overweight. This is something I've struggled with since middle school, being right on the borderline and overweight and like 89th percentile, which has significantly impeded my socialism and self-confidence. I just got a 24 Hour Fitness membership, and for the summer, or at least over the summer, if not starting now, I want to really get in shape, no matter the cost. I'll give up junk food, soda, and the likes. I'm wondering what the best fitness workout would be at the gym. I've decided that I'll do a 30-minute regimen on the treadmill to get the blood pumping, but from there I'm not really sure where to go. If anyone has any personal experiences related to this, I'd love to hear them, in addition to any advice. Thank you.