Okay.
First let's get your account in gear. What two roles do you plan on maining the most? Don't fret over it too hard, I just need something to help get a frame of reference for the next suggestion. Get Tier 3 runes. If you only plan on spending IP on the game, focus on them first. You can afford to get new champions later due to the champ rotation, but you can't afford being objectively weaker than everyone else in the game as you enter. Get runes that complement whatever role you plan to play. Again depends on the role you plan to play, but a completed rune page for your primary role will go a long way. I also recommend you understand how Masteries work, but honestly it's mostly common sense, so I don't want to hold your hand there.
I want to save the rest of my advice until after I know what you want to play, but here's generally the golden rule: Just spend more time on the game. Really, experience is going to be a greater asset to you than any video or typed out explanation really could. Just try to slow down your mind as you play and take notes of what's happening. I don't care how simple it is; you can go as far to think "oh, she bought AD, she hurts more", as long as you are observing the game and thinking about it, eventually it will add up and you will benefit from it.
