Run side missions and encounters to get up to 30, I can't remember if repeating main missions give you the XP again prior to end game. Or if you can get a partner who's at the end game already, join their instance and let them carry you through acouple of missions to hit 30 quicker. (If PS4, feel free to add me my PSN ID is my username here).
Once 30 you have access to the world tiers, you want to run dailies, HTV's, farm the open world bosses (the likes of Bullet King are back if you remember that) to start getting better gear. Keep trying to up the world tier to get better gear score loot. Again, if you have a partner team with them in world tier 5 and let them pull you through the farm to get the highest gear score loot (256).
IIRC Underground DLC requires level 30 (someone correct me if wrong, it's been a while), however Survival only requires the base of ops to be unlocked so you can hit that straight away if you do buy it.
In my opinion, the season pass has been worth it. Since the balance and fix came in 1.4/1.5 and Underground had it's levelling requirements alterd (levelling is now quicker which equals quicker sealed caches) it is a real contender to farm for loot even on Hard rather than Challanging. I've had afew runs in Survival and it's a breath of fresh air after farming the open world for so long to have to play with different characteristics like halting your infection and keeping your hunger/thirst at bay. There is also a new faction in Survival called the Hunters who offer a different challenge to overcome.
World Tier is what designates your gear score items drops now. So you can play on Hard or Challenging but both will still drop the gear score items to whichever world tier you are in, Challenging just drops more of it.
It can also be 2 Gear sets 2 HE as well not a 1:3.
