The overall most important thing you can do when you are new: Not play ranked.
Ranked is horrible, the players are horrible (compared to normal draft), and you will learn much more slowly, especially since you will not be learning from observation. The environment is also vastly more toxic and tends to have younger, stupider and angrier players.
Pick a couple champions you like and are good at, that cover at least two rolls and complement each other (if one is countered, you can pick the other in most cases).
The most important things nobody ever mentions are not doing stupid things, and not being greedy. EVERY bronze team rushes headlong towards the closest visible enemy and either dives or trades, and stays there until they all die to an ambush. People running in circles in the jungle are not prey, they are bait for a trap. Always.
Pull dragon out of the stupid pit before fighting it, unless you like giving free dragon to the other team.
On that note, the team fighting dragon/baron is at a disadvantage, never start the fight if the other team is MIA, unless you know it will end within seconds and you also know the other team does not have time to react/group.
The defending team is also almost always at an advantage, do not push down lane to fight if you are consistently loosing teamfights, force them to come to you.
Wards, use them. Upgrade to farsight to catch teams at dragon, baron, or sneaking around jungle. Or just to check brush without getting wrecked.
If something fails once, it is not likely to work again - do something else.
