How do I get better?

I've been playing League for a long while, just got to level 30 last night. (not enough champions for ranked play) However, I feel like I'm really lacking in skills but don't know how to improve. I played with my friend's friends the other day and they were so much better than me, even when one of them was lagging quite a bit. I've been told things like I spread my choice of champions way too much and I don't buy appropriate items for the situation. Any suggestions on how to improve please?
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