Fastest way to level up in game to level 18??? Need help

This should be easy to find, and maybe you can give me help on how to find my answer as well. When I search the forums and YouTube, the only answer I get is how to level up to 30. I'm looking for the best and fastest way to level in game to the highest level.

I've heard about standing near minions which I've tried. I've also had the most champion kills and minion kills on either team, and a tower kill, and been one or two levels lower than the top guy. I don't understand that.

Help please.

Thanks!

Edit: It would be nice if someone actually answered the question instead of saying "get to grinding" or asking me which lane I play.

Once again, how do you level fast in game? As I said, I've had top minion and champion kills with a tower kill mid game, and been two levels behind. So there must be something more to it.

Once again, thank you if you're actually going to answer the question. I really do appreciate it.