You will probably die a lot in the Dark Zone, especially on your first few forays. Unlike the rest of the map you can't fast travel to locations you've already found, but, much like the rest of the map, if you die you'll have to respawn back at a checkpoint. In New York at large this means a safe house, but in the DZ it means one of the checkpoints around the edge (there are also safe houses in the Dark Zone, but they're not marked on the map for you to head towards, so to start with the checkpoints are you only option). Try and open up a few of these before you head deeper into the middle of the DZ, because if you die out on a limb it's a real chore huffing it back from further out. This means doing a bit of recon round the edge to start with, but it's no bad thing.
This has been mentioned before, but the DZ is in a much higher difficulty tier than the rest of the game. The enemies you'll meet are higher level, will all have shields, and move in bigger groups. You need to be level eight as an absolute bare minimum, and even then you're probably still going to bite the dust, unless you're just really good at the game or something. Level ten is a safer bet, but even if you can take on the NPCs comfortably that's not accounting for other players.
As mentioned above, the NPCs in the DZ hang out in larger groups, and set up at some intersections to catch anyone passing. In many cases they'll be set up at the same intersection every time, so you can use this. If you're in a fight with some roaming Cleaners, for example, and they get you down to the wire, you can bolt off towards the nearest blockade of Rioters, and then watch the resulting firefight from a distance, before running away. You can also use the same technique to hopefully distract rogue agents chasing you (or regular agents, if you've gone rogue).
One of the joys of the multiplayer in The Division is that it's incredibly easy to quickly to jump into and out of other groups of players, but this is trickier in the Dark Zone, because the odds are pretty even on the other players trying to kill you. The NPC gangs are easy to get away from if it comes to it, but real people are a different matter. Try to take at least one other squad member with you, just as backup if things get a bit tasty, like. They probably will.
When you head into the Dark Zone there are a few ways you can communicate with other players, like the emotes, so you can salute or wave, or start doing jumping jacks in place to indicate that you aren't a threat. The easiest way, though, is to make sure your mic is on and you have voice chat enabled in your settings (it should be on by default, but worth checking anyway). The Dark Zone has proximity chat, so when you're close enough to other players you can broadcast a friendly hello and an offer of peaceful truce directly to them. This may or may not be ignored, of course, but it's a faster way of clearing up everyone's intentions.
Alright, once you've gone into the DZ and killed a few people you'll notice that you're now levelling up in a separate way to your otherwise regular level. Your DZ rank doesn't mean much in the larger game, but there are items that you can only buy from vendors at the DZ checkpoints and their usage is gated by your DZ rank.There are also loot crates scattered around the DZ which are locked to anyone below a requisite DZ rank – there's one in the first area of the DZ that requires you to be rank 30.
DZ rank goes up as you perform successful kills and so on, but you lose DZ experience points whenever you die in there. You also lose more if you go rogue and get killed, as a punishment for being a filthy turncoat (and other agents get more if they're the ones that take you out). Basically, you'll need to decide how important the DZ rank is to you, because the more often you go rogue the more likely you are to lose a rank.
This means all the different food and drink, ammo types, and grenades. In the main game you can never use any of these and get away with it quite comfortably, but in the DZ you're going to need an edge, and being able to boost your damage or flashbang the other guy when he's not expecting it can be that edge. The flashbang in particular can be very useful, and very annoying if you happen to be on the receiving end.
There are enough extraction points in the Dark Zone that you're never catastrophically far from one, which is great, but when you get there don't just rush in and fire a flare off straight away. The extraction points will often have enemy NPCs or rogue agents hanging around spoiling for a fight, and even if there aren't on your approach there probably will be soon, since when an extraction timer starts anyone nearby will be alerted to it.
Take your time approaching the extraction point. Make sure the area is clear, and then prepare to signal the helicopter. If you're in a squad then get a few people to take up a defensive position around the area. Make sure everyone's skill cooldowns are done and they're ready for combat, and keep an eyeball on any strange agents in the area in case they decide to pull a fast one. Then fire the flare for the helicopter. Be smart, basically.
A lot of the points in the guide so far have been geared towards agents who play fair, but the numbers show that about half of you reading this will not be that kind of agent. Attacking other agents for their loot is known as going rogue, and if you go rogue then you'll be marked as an enemy and a viable target to any other players you come across. So if you're going rogue you need to commit. Plan that you're going to turn traitor beforehand, and build your squad with that in mind. Go hard on the DPS, because players are (generally) smarter than the AI, so you need to take them out before they heal up, but take a medic along with you as well to patch yourselves up after a fight.
Yeah yeah, it may be easier for rogue agents to just take up position around the extraction points and camp there to take out as many incoming agents possible with the minimum amount of effort. Counterpoint, though: is that really that much fun? Don't answer in the comments, that's rhetorical. Look, it may involve less huffing it around the DZ to find victims, but it's also what one would technically term a 'dick move'. It also doesn't hold up to much examination. Yes, rogue agents need to extract their ill–gotten loot just like anyone else, but it's actually better to keep a checkpoint within easy distance and doing planned runs between the two. Dying as a rogue agent carries harsher XP penalties than it does for regular agents, so being able to cheese it to a checkpoint and out of danger is a boon.
That cheesing it part we mentioned above? That's easier if you take the time to explore the area you're mooching about in thoroughly. It gives you a tactical advantage over agents that don't, both if you're attacking them (because they won't know the terrain and you will) and if you're running hell for leather to escape trouble, because you'll know the dead ends to avoid and the shortcuts to take. If you know the area well and you're smart about it you can lose the other players. Or at least survive long enough to get the hell out of there, which is sometimes all you can hope for.