I haven't played since warmind. What am I supposed to be doing, so lost.
You complete all unfinished expansion campaigns and then do activities that have powerful/pinnacle gear drops to continuously increase your power level until it reaches a high enough level to play in the current end game meta.
Bounties are basically essential for leveling faster so always grab as many as you can.
Very grindy, but Destiny is one of those games where you either enjoy the grind or hate it.