I'm currently downloading it. Explain everything and I mean EVERYTHING. Define what everything is and all that. I got a little knowledge on it but not enough. I know a lot about Minecraft so you can use that to relate to it (even though MC is nothing like LoL)
What is League of Legends all about?
Its basically this RPG game. Your goal is to destroy the nexus. Every week there's free to play champions. There's also skins which you buy with RP. You can buy champions with IP or RP. You have to destroy minions but you also gets some minions. There's also enemy champions to buy. There's a shop in the game to buy items for the champion your place. There's AP which is long range ability power. AD is attack damage which is mostly melee champions. Support is to help your teamates. You have 4 skill with extra skills you unlock in diffrent levels. Your last skill is the ultimate which you can improve throughout the game. You can improve each skill with each level. In the game there's also monsters to kill such as wolves and golems. You mainly killl them for buffs or you going jungling which killing monster in the woods. You would also like to Gank which is go from one lane to another and surprise attack someone else in a diffrent lane. More is in the tutorial when you start the game
Multiplayer game. There are 100 or so champions out right now that you can select for each game you play. Each game is roughly 30 minutes. Your goal is to destroy the enemy Nexus. You and your team need to cooperate with one another and fight together in order to destroy the enemy nexus.
You won't have enough money to buy your own champion when you start but each week there are several free week champions that you can play then one you win a certain amount of games you'll earn enough IP (LoL currency) to buy yourself a champion.
Just google some youtube videos and you'll see what I'm talking about.
Here you go. Its is official game guide made by developer Riot Games. Here is every thing you need to know about League. If reading is not yout thing this forum post contains video explanations, but they don't cover every single aspect of the game. Just before you sing up make sure that you are making account for right server. Example if you are from America then you want to make account on American server for better performance.
http://forums.na.leagueoflegends.com/board/showthread.php?t=556519