first things first get all max tier runes. until the rune update is released you need all tier 3.
secondly - figure out what champions you're most comfortable with. melee or ranged. defensive or offensive. abilities or basic attacks.
third - learn ALL of the controls. not just the basic controls. ALL of them. there are some unassigned commands that you can keybind to make your life easier.
4th - make sure you have decent internet speeds. most high level players are around 20ms in game. i play on 40ms
5th - there are currently 5 positions on a team that most players agree with. those positions are tank/fighter, jungler, mage/assassin, support, marksman. top, jungle, mid, support, bottom. these arent the only positions, but they're the most accepted and you're less likely to be griped at by teammates if you conform.
6th - kills are helpful
7th - deaths are detrimental
8th - objectives win games. objectives are drakes, herald, towers, baron, elder dragon, inhibitors, and the final objective the nexus.
9th - drakes, and towers are permanent disadvantages when lost. so dont lose them.
10th - you earn more gold from last hitting minions than you do from slapping the same under leveled champion around. you earn more gold from killing a champion who's beating up on your team than one who's being beaten up by your team.
11th - xp is best earned by standing in lane. minions grant the same amount of xp regardless of last hits or not. killing jungle minions grants xp. killing players grants xp, but most xp comes from the lane minions.
12th - use TAB to scout your opponents and look at jungle camp timers.
13th - wild pokemon hide in the tall grass. never go unprepared to fight.
14th - remember to get a trinket at the start of the game.
15th - you dont have to buy an item at the start of the game.
16th - learn all of the stats and how they work with your champs.
17th - dont let anyone tell you how to play.
18th - use the mute button in the TAB menu to retaliate against harassment and then report that person after the game. i dont want to play with jerks anymore than you do. let's work together to weed them out. dont set yourself up to have your account deleted by typing back insults.
19th - dont chase singed
20th - buy zhonya against karthus
21st - dont go near heimer
22nd - mundo goes where he pleases.
23rd - hide yo wives hide yo kids hide yo cuzins cause thresh be kidnapping everyone up in here.
24th - patch updates roll out on tuesdays every other week. so if you're on when they roll out expect to lag. a lot.
25th - never fall to bronze in ranked or you'll be stuck there for life.