Need help...

I'm a player who's bad for joining and leaving WoW continuously, I stuck to it for at least 6 months last year since I had the Recruit-A-Friend advantages but I never really learned how to properly play WoW myself. Putting that to one side, I've decided to give it another run since I've got a lot of spare time on my hands and it is a fun game once you get into it, but I'll need some tips.

I want to know an ideal realm to join, the best side out of Horde and Alliance, best class and character to be.

Thanks in advance y'all.