yeah. i'm now competent enough so that i can just walk into a normal game without worrying about feeding.
but before i learned cautious gameplay and general mechanics.
i'd play a custom game against bots. just to familiarize with abilities. and i'd pick up a build off mobafire or one of those sites. if u get the build part out of the way, then it's a lot less to focus on.
next i'd recommend playing a coop vs ai. don't worry about lane assignments. just pick ur champ and go and familiarize yourself with mechanics a bit. the coop ai is much better than regular custom.
then play a normal. if they start raging at you. just ignore user/mute them. if its not advice and just raging u dno't need to listen to that. report them for being toxic at the end of hte game. at level 12 you have nothinjg to be ashamed of. the guys raging at you are probably not that good themselves or are smurfs who may have been banned from regular level 30 play.
try and learn from your teammates. but don't get upset by their rudeness. it's going to happen. stick tough.
oh and play arams too. they are great for learning mechanics.