Garen is the lowest skillcap hero in the entire game. If you can get the IP to buy him, do it. He is exceedingly easy to play. You simply run up to people, and press Q, W, E, and R when their health gets low. The build for him is usually Sunfire/Mercs/Atma's/AD+Crit. In Ranked games, you would use Garen to counterpick teams relying on AD carries too much.
Mordekaiser is another good hero to play, as he's also pretty easy to use.
I play Ashe in ranked, and it actually requires a lot of skill. You're a fragile champ with no escape mechanisms, so people will gank you left and right if you're not careful.
If you don't have much IP, Nunu is fairly easy to learn to play, can lane or jungle, and is also great at all levels of play (best jungler in the game). Epik used him in their famous first game against Counterlogic in the North America Finals, and Nunu pretty much won the game for them. Bryden7 is one of the best players in the world, and his most-played champ is Nunu. Believe it or not, Nunu is a good champion.
AP Sion is another very easy champ to play, as long as you know how to last-hit well, since he needs a lot of farm. Sion is the best laner in the entire game, though, and farming is easy for him, so don't worry.
Referring to your questions.
Gank: The use of a proximately superior force to defeat an enemy. Basically, one guy coming in behind you to hit you with his buddy.
Last-hit: Landing the killing blow on a creep, giving you more gold.