As someone who plays both a ton, I'd definitely say League is A LOT more competitive. Dota's community is just as bad, if not worse but it's consistent. I'd say League's gets a bit better if you manage to climb up out of lower elos. That's not to say that both aren't a blast!
Each side has it's own jungle. GENERALLY the carry is farming in the lane closer to their side of the jungle. It's called the safelane. Carries are usually melee guys who build right click items. Teams tend to have two supports. One who sits in lane or gets a tiny bit of farm. The other usually roams or is a ward bitch. One support can be a jungler who gets a big initiation or big team support item. The mid laner is someone who can roam well. Jungler doesn't have nearly as much map pressure as in League so the mid laner is 'expected' to roam. Opposite of the safelane is the offlane. Usually it's someone who is able to either safely farm like Broodmother or Bounty Hunter, safely get exp like those two or who can roam like Nature's Prophet. The meta isn't nearly as strict. Which results in a lot of trolling unfortunately. I'd say play with friends until you're out of the trench.
As someone who played League, it was kind of lame coming to Dota playing clunky/boring heroes. So a lot of the simpler characters like Wraith King or Slardar I avoid. But there's A LOT of heroes who make you think in ways you don't in League: Invoker, Lone Druid, Meepo, Rubick, Phantom Lancer, Storm Spirit, Earth Spirit, Faceless Void, Nature's Prophet and Broodmother are all worth playing at least once.
Play the game. Learn the purpose of the item rather than whether or not it's good on the hero so you can make your own judgements. Torte de lini has the best guides you can access in game.
Helpful knowledge is just from experience. Try spectating games and seeing how they go. Ask tons of questions and you'll learn!
I played A TON of Rubick when I first learning. His ultimate is Spell Steal. You steal the last spell a hero casted when you use it. So it forces you to learn what each ability does as well as learn one of the best supports in the game :D