Best tip IMO: take a class. At fifteen you should have numerous sources for structured education. Possibly your high school has a photography or photojournalism class. Any junior or community college will offer such classes, you may be able to audit the class even at your age. Most camera retailers have some sort of class you can take for a small fee. They are typically very short and only get you the basics but, they are a start.
If you insist on going this alone and being self taught, find the best course for your interests. Ask for suggestions for courses directed towards, say, portraiture, landscape, fine art, etc styles and get on board. You don't need lots of equipment nor expensive equipment. In fact, I think being limited at first to basic gear will force you to think more about what you are doing before you take a shot. One local instructor here in Dallas teaches only film and with low cost, used cameras to keep the students focused on the process of photography rather than the equipment for photography.
Once you've decided on a course, stick to it. The many issue I see with self taught students in any subject is they are too diffuse in their interests. They want to know it all and they want to know it now. This causes them to skip around from topic to topic and lesson to lesson with this teacher and that instructor. IMO that wastes a lot of time because there is no structure to the plan. A very good instructor will understand not only do students learn at different rates but they learn best when one topic is being introduced to support the ideas of the next and the next lesson. A very good instructor lays down a solid foundation first. Therefore, find a good course for your area of interest and then follow it page by page and lesson by lesson. Even when it gets boring, that's part of the foundation required to build on. So stick with THE PLAN!
Having an outside critic is helpful but not completely necessary. You are still rather young for the work and very inexperienced. However, if there is a working photographer in your area willing to take on a young apprentice, possibly this might be a way to learn while earning your keep.