Start building a portfolio: a collection of your work that showcases the depth and the range of your skills.
By depth of skill, I mean: whatever your strengths are, emphasize those in a few key pieces. Maybe you really want to show how good you can do, say, environments. Maps, landscapes, scenery - so put the time and effort into making a couple of really good landscape pieces. Do it over and over until you find one that you're really proud of, that shows off "I can do THIS really, really well." If you don't think you're there yet, that's okay, you still have time to practice. Keep practicing.
By range of skill, I mean: don't just have one kind of art in your portfolio, if you can draw more than that. Again, maybe your best thing is landscapes, so you have a lot of really good environment pictures. But you're like, "Hey I can also do good costumes/clothing/fashion designs too" because actually drawing the textures and folds for different fabrics and leathers is a good skill to show off. So you want to include a variety of pieces to show that you can do more than one kind of art.
And again, I cannot emphasize this enough; keep practicing. Do not get discouraged if this is something you're passionate about. You're young, and maybe League of Legends won't really be around like it is now in another 5 years when you turn 18. But you know what there will be? Games - awesome games, beautiful games, fun games. Maybe there will be a game even better than League of Legends by then. And then you can try to get a job with them! But keep at it, okay?
Seriously, if you ever feel like you want to give up, take a look at this. This guy started off as a not-so-great artist - certainly not professional level. But he practiced and practiced and now he's amazing, and he has even done art for Wizards of the Coast (the guys who make Dungeons & Dragons and Magic The Gathering)
https://pbs.twimg.com/media/CJOl6OIWUAAmSBy.jpg
His name is Noah Bradley, google him sometime to see what he does now, but he had to work to get there.
Keep your chin up and don't let the world get you down.
[slayer-pantheon-thumbs]