Oh great, another thing to learn. I do try to point out some of the really obvious things when I see them like leaving a lane empty at the start (and apologize if I'm the one making the mistakes) or ping for assistance if needed but since I'm new to the game I don't want to say anything wrong.
Another question: If I want to level up the free heroes I should probably play a couple of training/bot matches first right? Doesn't seem like a good idea to go into quickmatch without playing the chararcter first.
98% of the people you are with are either brand new or clueless as well so I wouldn't worry. They are going to do one of three things: (1) be happy someone told em what to do (2) ignore you or (3) get toxic. There's basically no avoiding 3 at the MMR you start at so its not even worth worrying about. Just mute them, report them and move on.
As for leveling heroes you're unfamiliar with, that's up to you. Odds are by being here you are already well ahead of the curve even though it's a your first moba. Quick Match is the casual mode, but naturally people still wanna win and may get salty if you visibly have no clue. I'd say you'll be fine though.
My only caution would be to consider that your first 20-50 QM games have huge weight, and your MMR will stay malleable up to and beyond ~100 games. You want to be as far away from starting MMR as possible (if possible) by 100-150 games in as your game experience will improve drastically if you can do so. It's a catch 22 in that way...you learn more about heroes in QM but you are also eating up your early games on a hero you may be awful at.
It's not a huge deal, but I know a few people who would've loved to know better ahead of time (same goes for starting HL as well). As I said, you're probably well ahead of the curve by being on a forum like this and watching videos already so do whatever you feel is best. If you want to keep track of how well you're doing, upload your games to
hotslogs.com. The MMR there will give you a ballpark, but the main draw is seeing how well you are doing on certain maps, with certain heroes, with friends, and with certain talents. There's lots of info - including other people's info that you can use to better your play.
Bot games are good for dailies though if you get win 3 games or something like that and hit a dry spell in QM.