Sucks. It's putting me off the game tbh. 3v3 is no better. I just had 2 guys go afk several times in the first half. I can't play with lvl 30s because I just get owned.
You get the odd random good game, where people aren't retards, and it's not SO bad at level 30 than it is your level range. Easiest way to win though, is to master a champ and carry the game if it happens =P Far too many trolls in LoL though.
Just keep learning, practicing last hitting minions etc,.. you'll get hang of it. Most important thing is knowing when to go, and when not go. Learn that for your champ and you'll be fine. Oh and don't get greedy. Ever. Not worth trying to get that kill when there's a chance you'll die doing so, a 1 for 1 death exchange is bad (unless ganked and you get a kill out of it, that's very good :P) always try to get a kill then secure it / recall on low hp, even I get greedy now and then thinking I can hit them one more time for another kill, but they out-smart me or I just don't do the damage.
Also, if you're using heal, best way to lure someone for a free kill is get low health, bait someone into tower diving... heal, laugh, free kill.