Depends on what you enjoy.
If you wanna raid, start reading raid guides and watching videos, start grinding ToC5, FoS, PoS, HoR both normal and heroic versions for best gear you can get before raids. Search the AH aswell for good pieces. After you're well geared, join a progressive raid that doesn't need achievements or gearscore and learn with them.
If you wanna PvP, start reading PvP guides and just join lots of BGs and just PvP whenever you can. You'll figure out which abilities are best used and when on your own.
If all of that is too much work, you can always go achievement hunting. It's pretty casual and chill.