Welcome back to GW!
As you've come back from a lengthy absense, I would suggest heading to the
GW Wiki and reading up on some of the major things that have happened. Many, many things have been added to the game over the last 12 months alone, and the wiki is the best place to go if you need to read up on things.
I'm assuming you still remember the basics of gameplay, although if you need a refresher course there's a half-decent tutorial for new characters starting in Nightfall. You'll also see two new professions appearing at character creation if you start a Nightfall character; have fun experimenting with them.
If you cast your mind back to Prophecies, you will probably remember that you got a lot of skills as quest rewards. You don't get these free skills in Nightfall, and have to buy them all after you've picked up the few free ones you get during basic training. To compensate, a lot of quests give you things like Battle Commendations which you can trade in at quartermasters for adventuring supplies (ID kits, salvage kits, chest keys, bags and the like), which saves you money that you'll need to expand your skill collection.
