Heres some tips from a experienced point of view, sorry it's so long and blunt and to the point, bear with me xD
Mobafire has all the champion builds, and guides, if you search and champion and then guide into google you will see mobafire guides, look at a couple of them, decide what works best for you, they have not only the items you build, but the abilities you max first, and lots of tips and tricks and rotations etc etc, works wonders, and since you played Dota, you know the basics, all i can say is, don't be that one guy that goes in over and over trying the same thing expecting a different outcome, they call this "feeding" xD know when to base, know when to passively farm if you have to, and know when to wait for help.
Annnnnd remember, don't flame, This game has the absolute worse automated ban system EVER, it will take number of reports over the actual person who needs punished, so if people start to flame you, hold tab, and mute them, don't even start with them. And lastly, play to have fun, this community has lost its way a long time ago, they think everything is about winning, and flaming anyone who gets in the way of that. So just mute them and you wont ever have an issue :3
Solo queue no longer exists, get friends quickly to queue up with, or you won't have a good time, sorry to be so blunt, but that's the sad truth, if you queue solo, you go against trios with voice chat and actual team compositions, which make it impossible to have fun and learn new champions you have never tried, especially since your team will more than likely be filled with people who just want to play whatever champ they want that goes in a certain lane xD, and they won't be coordinated, friends makes league a whole lot more fun, and a whole lot easier to win and get good on champions, its simple, you can't learn a champ or a lane if the enemy team is voice coordinated ganks and dragon calls and such, so it's best to be on the side that has that advantage, and then you don't have to worry about messing up because friends don't care, they just wanna have fun!
Alrighty sorry for that long ass reply and bluntness about some flaws of the game, just trying to prepare ya for whats ahead haha, good luck and have fun! and Remember, no flame![slayer-jinx-catface]