In a game like this there are a lot of things to learn, and it takes time. Finding your main might be easier than you think, when i started i just picked Ezreal, and then stuck with him for a very long time. Decide what you like, ranged, melee, tank, fighter, mage, support, jungler. Learn that one champion, and wouldn't hurt to watch a stream or something to see the champion in action. When you feel like you got that champion down, try a new one, maybe another type of champion completely.
The last hitting part takes time, and some champions are better than others because of abilities and such. You just need to learn what the HP bar looks like when it can be last hit. I'd advice you to play normal 5v5s with friends, or go vs AI solo until you feel comfortable with the champion of your choice and you feel like you can last hit better.
Best of luck to you bro!