Honestly, I've only been playing for a few weeks. I was terrified to play my first pub game because everything I read was "You suck" and "You'll never be good at this game" and "Everyone will troll you because you suck so bad".
So I spent like 3 weeks playing bot games, learning a couple of heroes pretty well. I watched a bunch of videos and read a bunch of guides.
Then I did my first MM pub game. And played with people who didn't even bother getting boots. And I stomped them.
So now I'm not scared to play pub games anymore.
The thing is, though, that at the very, very low skill games, there's no teamwork. There's no coordination. So if you spend time playing bot games and you know what you're doing, playing a support hero is wasted. Just play a carry and steal all the last hits and roflstomp your opponents. With the right carry, you can seriously just own the fuck out of their entire team if they're bad enough.