Rune: Choose wisely; They're meant to boost your chances in a level 1, 1v1 battle and sustain you in the long run. Attack Damage Carry runs usually include armor penetration, life steal, attack damage, attack speed, armor, and magic resist. Ability runes usually include magic penetration, ability power, mana increase, and spell vamping. Tanking requires health, armor, and magic resist.
When 5v5 summoner's rift, top lane includes tanks and mages, whichever is the preference, middle lane is usually mages, and bottom lane is for supports and assassins. The jungle is something to be handled with experience as it is a bit more difficult to choose a correct route and proper equipment to keep leveling with your team without getting owned by the monsters or by a possible invading champion.
When 5v5 ARAM, it's all one lane with a random champion making it pretty simple to just have fun experimenting with the champions available and what items to use in tandem with their abilities.
When 3v3, it's best to go with players that you're familiar with to go in strong and come out winning. It's two lanes, a jungling area between lanes including two alters on each side of the field that offer boosts for the team owning them.
After you own 16 champions and reach level 30, you can participate in ranked matches to gain some rep in game, but this will be very stressful since most players are hanging on the edge of their seats waiting for someone to make a mistake so they can curse you for failing to do your job and accuse you as being the reason "why we can't have nice things".
Besides all that, welcome to the game and don't let them downsize you. GL HF