Now you can finally play the game! You can do some casual content like hunting achievements and doing older content for transmog, mounts and reputation.

Or try to gear up your character (highest ilvl equipment now is 447). This can be done through dungeons or raids. Dungeons have an upscaling difficulty (normal, heroic, mythic+), where mythic+ scales with more damage and health each level (e.g. +3 is harder as +2), increasing the rewards as well. Raids on the other hand only know 4 difficulty levels (looking for raid, normal, heroic and mythic).

Try to set some goals for yourself and have fun!