You'll want to look into a career around "Game Design". Before anything though, I'll have to warn you that it's not an easy life. Game Design is full of ups and downs and grueling work hours for not a whole lot of pay. It's not for everybody. However, if you think it's worth it and still wish to take the plunge there are a couple ways into the industry.
Riot is pretty well known by now, which means they won't just be taking anybody. Before you work for Riot or anybody like Riot, you'll need to build up an impressive resumé. Remember, no one is going to take an "idea guy" so you have to provide more than that. You'll have to build up skills that show you can be an asset to a development team, like coding or art. I suggest you figure out which you're most comfortable with and start working on it. If possible, you could be strong at both which would make you much more valuable. Alternatively you can try downloading a basic game making program (e.g. GameMaker) and try to make a few small games yourself and prove how strong your design skills are.
Either way, the more you can "prove" your worth as a developer the more likely people will take you. The sooner you start creating a portfolio of your work, the better. 15 years old is still pretty young. If you start working hard now, you'll be pretty ahead of the curve.
Unfortunately, it could still be many years before you're ready to help design or develop games. It'll take even longer to get into a specific company you want and then it'll be even longer before they let you touch one of their main attractions (e.g. Champions).
tl;dr What you're asking requires years of unwavering dedication. Start working hard now if you really want it. Be conscious that things may not go the way you want even if you pour in years of work.
