Just downloaded this yesterday after playing a game with Skillmister and MoNkeE.
I played one game last night, 5 vs AI, and we got slaughtered lol. I have no idea what I'm doing though to be fair. I always pick melee characters in games, like Diablo, WoW, I always go for the tank/barbarian type class, so went for Fiora as she seemed to be melee. But wow, died so many times. And went for the middle lane which I got told this morning was my first mistake.
Going to try Ashe tonight as that's who I've been recommended to try to learn the game a bit before trying to play melee
The game has an extremely high learning curve, mainly because of the meta game (the way in which the community agrees is the best way to play) / lanes / various champion types and abilities.
So expect to die a lot to begin with.
But it certainly is a fun game, ashe has a lot of damage, but is squishy, if someone is hitting you, don't stand and take it.
I'd suggest you try some tanky people, for a few main reasons: You're not expected to get all the kills, you will survive longer simply by being the tank, the enemy team will not focus you because you're the tank, again leading to longer life.
Good luck
EDIT: i won't add much, because you could type pages and pages to cover everything, but a quick word on the meta:
Generally agreed that the following layout is obeyed,
1 person top - Often AD / Bruiser or tank. Normally melee
1 Person jungle - Plenty of jungle champions, who's abilities vary, they kill the neutral minions and jump into lanes to help their team. They level a bit slower but get more gold than lanes (generally). Watch out for being attacked by the enemy jungler (wards can help. If you see them, tell your team, if you need help tell your jungler)
1 Person middle - Generally an AP caster type. But doesn't have to be.
2 People bot - Normally 1 AD carry (carry means they need help early on getting the kills, but they snowball and get very strong with each kill / item, making them extremely powerful lategame if they've been given a good early game) and 1 support - Not someone who goes for kills, goes for support items (like randuins omen / frozen heart / shurelias / mana manipulator [and whatever it builds]) to buff their teams stats / lower the enemy teams. They often have heals and silences to help their team.
Sometimes you get 2 people bot where one is a carry but the other isn't quite support, watch out for big damage combos in this case. (I.e nidalee 'support', or nunu.)
Anyway, gl!