1. Go Introduce Yourself to Eris
Eris is the main NPC that facilitates the events and quests of The Dark Below. Before you do anything, go visit her in the tower and take all of the bounties that you can. As you’ll notice, she has a new reputation rank and lots of new items to buy. Be sure to visit her every day and slowly but surely you’ll be able to raise your reputation with her.
Side note: This is an informal tip because it doesn’t apply to most longtime Destiny players, BUT you should be at least level 26 or so before tackling The Dark Below content. The missions start at 24 but increase in difficulty each time. Check out this guide if you need help getting your light up.