Start with what you will play
Choose your realm ruleset, then decide whether you want to level alone, run group PvE or play PvP. A strong choice in coordinated raids may feel slower when you are learning alone.
Read a tier list like a test result
Check the patch, ruleset and activity behind the ranking. Pick one role you want to practice and use Blizzard’s class and specialization descriptions to compare how it plays before committing.