Returning player questions

I haven't really played D3 since the RoS expansion launched. I don't really know much at all about D3s current seasonal content funnel. I've just been doing random adventures and mostly running the Nephalem rifts to level. What should I be doing? And outside of the alter what's the seasonal mechanics? I'm level 55 atm on Demon Hunter. I guess I'm just asking for some direction because atm I'm just leveling doing whatever and feeling like I'm being super inefficient, possibly just missing the seasonal mechanic.