I sorta was in the same boat circa about 2 months ago. Now, I'm almost done my legendary ring quest and am doing pretty solid thus far. Here's what I've found:
The game has been homogenized quite a bit. So if you're an older player, things might feel a bit "off" and should be easier than in the past. This might just be my playstyle or whatever, but I find Monks and Death Knights are the only classes that don't feel watered down.
Jumping into Draenor will get your caught up on everything for tradeskills. It is REALLy easy to get back into the game.
You'll be a doing a lot of stuff involving your garrison. You might be wondering what to get, buildings-wise. What you want to build depends on your playstyle (do you like PVE or PVP) and what professions you have.
I recommend getting the Draenor Treasures add on. There are a TON of treasures scattered across all the zones in Draenor and they give you a bunch of XP, gear, Toys and garrison resources. If you want to explore the world naturally, don't get this add on, but I highly recommend it.
I also recommend getting Master Plan for your garrison. It makes the Garrison a lot easier to manage and most people would say it is a must have.
There will be zone objectives you can do that grant a ton of XP. They are usually marked on your map with crossed swords and you should get a pop up if you're in an area that offers one. They're basically autocomplete quests, but grants a ton of XP. Also, you need to complete all of them to unlock flying in Draenor (along with a bunch of other stuff.)
Don't be afraid to run missions with your followers if they're missions for follower XP. Even if they fail, they will still get some XP and there's still a chance they'll succeed. The faster you can get your followers all leveled up the better the rewards you'll be getting (although, again, a lot of that will be changing once Legion launches.)
There is a ton of gear that will get you up to pace for LFR once you hit 100 and get Tanaan Jungle unlocked. Similar to Timeless Isle in Pandaria, if you were around for that. If not, basically, there's a ton of treasures around in Tanaan, as well as a bunch of treasure chests that can contain free gear.
Prioritize working on Flying. Look up the achievement Draenor Pathfinder. You can get a lot of the requirements out of the way on the way to 100. You can get away with a lot of crap once you have flying.
Other than that, don't stress too too much. If you have questions, feel free to hit me up or post in the sub, most folks are pretty helpful!