I'm trying to think of where I was when 15. I had a lot of interest and great motive to learn but information was lacking. I did have a PC computer and some very basic Radio-Shack-level electronics components. The internet and WWW barely existed, though there was BBS. The only sources of information I had was the library and bookstores. Library had some obsolete books, like "Program BASIC on the TSR-80". Bookstores had some weird titles like "Gadgeteers Goldmine Circuits for the Mad Evil Genius". Just getting good information on the basics was difficult.
2 things would have been extremely helpful to me at that time. One, would be to have a mentor or somebody I could ask questions about computers and electronics. Second, would have been real textbooks that explain the basics, like Ohm's law, Kirchoff's laws, truth tables, circuit analysis, etc. I think there is no real substitute to having this basic information. Even if you don't read textbooks cover to cover, having them as reference is very helpful.
My advice for the original poster, would be to find some mentor at school, maybe a science teacher, or join the school computer or electronics clubs if they exist. I guess internet forums and Youtube can fulfill some of this role now.
Second, would be to stock up on textbooks, first year college textbooks give the basic information, math, physics, electronics, programming, etc. New textbooks are absurdly expensive, $100-150. But, planned obsolescence is built into college textbooks, so you can find used editions on Amazon, 1-2 editions old, for $1-10. The internet is great, but I think having a nice collection of textbooks that cover the basics in a formal way is important. If you continue on and go to college you'll get that information.
I'd also second Dave's reply, get some breadboard, power supply, components and start experimenting on what interests you.