First up, welcome to the dark side.
These
articles here will help, some may be written in 5th ed, but they're still rather accurate in 6th ed. Also read up on
MSU tactics, you may choose not to run it at the end, but I think it's good to understand 1 of the basic concept about
DE.
All our ranged options are great, our shooting list is still considered top or high tier by many. Our close combat side, well, wyches took a hit, hellions lost their place now that beastmasters can move at the same pace as Baron, and then we have incubi for the rare high initiative AP2 attacks in this game... Oh, and reavers got boosted to be very playable(still costly) this edition.
As for wargears, Thor665's tacticas pretty much got it covered, but they could be a bit outdated, so here goes: Agonisers are still good, but you've gotta remember that it's only AP3 and won't work on some
IC out there. Venom blade is still its usual cheap amazing goodness, shredders are still poop, HWG got better... Hmm... There may be more that I've forgot, so ask if you're in doubt.
Good
HQ... Hmm..
IMO, Baron, Duke, Archon and haemy. Baron is just so cheap for what he can bring to the army, Duke a bit more costly, but the drugs bonus, poisoned shot bonus, etc just make him worth every points spent. Archon is very customisable and all around good. Haemy is a cheap
HQ who has a rather average profile, but he gives a pain token at the start and can bring liquifier guns...

The other
HQs are very good as well, except for may be haemy ancient or Kheradruakh.
Tips: Learn to accept that
DE are painfully fragile, we hit hard, but we take hits harder, our boats sink if enemy looks at them angrily(not hyperbole), learn to view it as "that unit ate fires and allowed my other units to live longer." instead of "I've paid x points and they didn't make it back, time to throw my game and a hissy fit HERP DERP". That said, don't just mindlessly throw your units to their doom, use covers to your full advantage, have low priority units stand between your enemy and high priority unit if you have to. And lastly, prepare to spend a lot as
DE isn't exactly a cheap army to play...

Oh, and you'll lose a lot because
DE is quite hard to play as well!