It is hard to answer because we have very different 15 year Olds out there with a big range of skills. If you simply want to do more programming you can do it all by learning online through examples and tutorials, make apps for mobile for free, or get a good handle on web-based apps HTML/CSS/Javascript which are easy to migrate across platforms. You can also pick up compilers for C and other languages for free.
I think the OP wants to learn more pure electronics and analog (or perhaps digital) hardware. In this case, going with Arduino and Raspberry Pi links you into a huge community of users and source code, plus teaches you some fundamental digital circuit designs. You can buy this stuff cheap and get kit bags from Element 14 and Ada fruit or tons of shops on eBay which give you assortments of components including some micros like shift registers, timers, etc.
Finally if you want just pure analog hardware, any beginner book is available in your local library. A breadboard and components even scavenged from junk electronics will do, but kits are so cheap anyways. Best to work through circuit examples building in complexity, transistors, LED, oscillators, etc. And a $50 CRO off eBay is a nice luxury, but even a basic DMM is a good start.
There is no magic formula. Just a willingness to learn. Along the way, you will make many mistakes. You will work through them on your own and with help from this forum. I would try everything since it is so cheap and just keep watching Dave and Ben, take things apart, put them back together, try to troubleshoot and fix things (best to do with small voltage battery operated stuff) and understand the fundamentals and the math and physics involved while you hack away more loosely experimenting by feel.