Quickplay is always going to be your best bet when it comes to non-comp practice since it’s really the only mode without any major gimmick. Gonna get even better for semi-serious practice once Role Queue gets put into place in about a week or so.
Again, learning heroes from every role is going to be pretty outdated soon once Role Queue comes in, since you can choose whatever role you like and be guaranteed to get it. You don’t really need to worry about having to play a different role as a backup.
Just in general try out all the heroes for a fair bit, play a game or two on each, see what you like and who you don’t.
When you do have an idea of what heroes you simply enjoy playing and have questions on how to play them or what team compositions they’re good in, always feel free to swing by here again and ask for some more specific tips.
But don’t force yourself to like a certain hero just because you think it’d be good to be able to play them. Sure, being able to play a tank with a big shield like Reinhardt or Orisa might be nice, but if you don’t enjoy playing them, why even bother in the first place?
Also personally, I wouldn’t really listen to all the people telling you to never touch comp, or to wait a super long time until you’re like level 100 or something. Sure, maybe don’t go in the absolute moment you’re able to, but like level 30 is probably more than enough. There are things to learn there that you won’t be able to learn anywhere else, as your teammates will actually take you and the game seriously.
And honestly, it’s the best mode in the game if you want to play actual Overwatch. Just remember you don’t have to prove yourself to anyone, if you end up in a lower rank, that’s absolutely fine. There’s absolutely no need to punish yourself just because someone tells you a certain number, be it your level or SR, isn’t high enough.