How can i (as a noob ) level faster within a game?

I just started playing league for a few weeks, but i've noticed that my level in-game is always 3 or 4 levels lower than my team mates. Also I have noticed that they buy lots of expensive stuff early in the game and i have no idea how they do that.. i try to kill minions as much as i can but still, the leveling goes very slow.. please help and thanks in advance
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