There are three types of money in The Division, which is interesting when you consider that the plot is a thinly-veiled criticism of material greed. Here is a game in which the "dollar flu" has crippled the financial capital of the western world and it turns out to be an MMO-like murderthon about getting more stuff. I don't know if the game is clever or if it just lacks self-awareness. Anyway, here's how to make loads of money. The Recharge Station upgrade in the Tech Wing will also increase any Dark Zone funds gathered by 10%, but that's less important early on.
Before we look at each currency and how best to earn, there are some upgrades you should consider getting first. The Medical Wing has an upgrade called Counseling which gets you 10% extra credits on all activities, which is an excellent early investment.
Dollar bills are out, on account of them being infectious, and "credits" are in. Blasting your way through the story missions is your first good bet, and doing them again on Hard will net you even more cash. The side missions are good for XP and the encounters are OK for supplies but neither is as good as story missions for making money. There are a few steps you can take outside of this to ensure you are always rolling in it.
Be frugal: Don't go on a splurge. That rifle suppressor in the shop you think is excellent will probably drop within the next hour or two of playing. This is especially true early on when you are leveling fast. Your weapons and gear will become outdated ludicrously quickly so spending lots of money on new stuff isn't advisable. If you are low level and you want a particular gun or piece of gear very badly, consider finding the side mission which offers the blueprint and working up to that. Then gathering the supplies and crafting it yourself. The crafting system is a bit overlooked by the game's tutorials, so we've explained it all here.
Always Be Closing: Sell your unwanted loot. There's a balance to be struck here between selling your junk and deconstructing it into usable materials but generally speaking if you are low-mid level you should sell, sell, sell. HOWEVER, some items are worth such a pittance it is better to deconstruct them. Know which road to take. You can mark everything you don't like in your inventory and then flush it all away to any vendor, or deconstruct it on the spot. Remember that better loot gets you better cash, so you might consider wearing gear which boosts your scavenging skill, which determines how much loot is dropped and how good it is. You can also up this skill by 15% once you get all upgrades on the Security Wing. But that is much later.
Go dark: The Dark Zone has some good shit. Extracting things from the DZ to your stash and then hawking them to your boys at home base is a good way to get money and new gear for yourself. If you plan to trawl the Dark Zone a lot, there is a Tech Wing upgrade called Recharge Station which increases your DZ funds (see below) by 10%, much like it's Medical Wing equivalent. However, considering the amount of regular money you can make from selling even crappy DZ loot, this upgrade is a lot less important than increasing the size of your extraction bag. For this reason, getting the Communications upgrade in the Tech Wing, the Dark Zone Ops upgrade from the Security Wing, and the Decontamination Unit from the Medical Wing are all much better investments. Having all three will increase your biobag's size from 6 to 9, allowing for more extracted goodies.
Dark Zone funds
This is the (almost) exclusive currency of the DZ. As mentioned above, the Recharge Station upgrade is useful. But the most common way to earn funds is just to form as big a group as possible with other players and go on an NPC killing spree inside the walls. Although the recent patch has reduced the amount of funds dropped by enemies it is still the most straightforward way of earning. Your real goal in the DZ is extracting good loot and killing the named enemies for Phoenix credits (see below), so DZ funds are pretty much accrued as a byproduct of this.
If you want to go rogue, in search of the dollah and goods from other agents, my advice is simple: don't bother. The punishment for dying while rogue is severe - a loss of thousands of experience points, thousands of DZ funds, any loot you have, plus any DZ keys you are carrying (used to open special chests). The latest patch made this less harsh but the rogue system is still finding its feet. What's more, the rewards other people will get for killing you are great. That red skull doesn't mean you're dangerous - it means you're marked for death. If DZ funds don't matter to you and you just want to be a rogue for the thrill and challenge: be my guest, we need more bastards like you.
Phoenix credits are the endgame currency unlocked when you hit level 30. They are for buying bling-encrusted weapons from the High-end vendor at your Base or from the special vendor in the north of the Dark Zone (he lives in an abandoned church and I think he either killed a priest or used to be one).
We've already covered the best way to make this kind of money in our guide to the endgame but the gist is this: do daily missions and tackle the four 'challenging' missions as often as you want (Lincoln Tunnel Checkpoint, Warren Gate Power Plant, Lexington Event Center and the Russian Consulate are all endlessly replayable on challenging). Usually one of the daily missions is set to challenging anyway, meaning you will get 50 Phoenix credits in total: 30 credits for busting through on the highest difficulty and an extra 20 for the daily reward. Prioritise this and do the other daily missions on hard. Then, if you feel like it, run through the others on challenging or the same one again. I find Lincoln Tunnel Checkpoint is the easiest of these if you are thinking of rinsing and repeating.
However, this can be very dull. A more exciting method of getting Phoenix creds is to form a pack and create a strong route through the Dark Zone. Team up and learn the best way to go from one named enemy to another throughout DZ01 to DZ03. These guys will drop 1-3 credits each on death. Don't neglect the subway tunnels, as there are named enemies and dozens of henchmen down there, ripe for the murdering.
If you are part of a co-ordinated team you might consider heading deeper into the DZ. A recent patch says the named level 32 enemies found in the northern sectors drop 3-5 credits but having been on recently I haven't noticed this. These fights can be brutal too, and given the DZ's tendency to spawn grumpy rioters behind you when you aren't looking, carries a bit of a risk.
There you have it. For more guides and tips on The Division, come this way