Changing between classes is good for the beginning, because you'll get to learn base mechanics of every single one of them and that will make it easier for you to get into the game.
Althou, you should stick to one spec / class at some point. After you do - best way to learn the class is practice, watching good players play the class and learning from it.
Don't just do bgs. Try some duels, so you'd know what to do in certain match ups etc.
Also a tip, that I would've loved to recieve when I started playing:
Get keybinds. This way you won't have to keep an eye on your action bars 24/7 and you'll be able to pay attention to shit that is going around you. Buffs, cc's as you have mentioned yourself.
Other than that, there's not much to tell you. Just try to enjoy the game :)