How i level up?

you level up playing wildrift , aram and so more gameplay modes. you can do trainings and practice

play tristana in co op vs ai intermediate and just mindlessly run down mid till nexus is down
you can get other things done while leveling up your smurf

You can always buy those starting bundles for certain roles, like Morgana, Nami and Leona + skins for support. They give you XP boost too, so just spam games and level up. I think it took me a week to level one account to 30, it's definitely easier than before.

The easiest and fastest way to level up is buying XP boosters and playing bots at low levels. Then you can always play ARAM