Do an Internet search for "training for a century ride". There are numerous training programs for that distance out on the Net. The distance you're looking at is close to that. Your time available to train is less than what most programs suggest, but you can adjust them to the time you have and just give it a shot.
You need to find "your sustatainable pace over distance", build up saddle time to prepare your behind for the "abise" it will take, learn to drink/eat to keep yourself going, and learn to handle the bike/repair it (flats, chain thrown, etc.). Figure out what you what/need to carry with you and how - ex., how many water bottles, cell phone or money for emergency calls, pump or air cartridge for flat fixiing. Also of importance, you need to learn about riding in hot weather and when to stop due to heat/dehydration issues.
If/when you have specific questions, be sure to ask them - here or elsewhere.
FWIW, at 15 years old, you've probably got more energy than I did when I started riding again and I did my 1st century only 2.5 months later.
Good luck.