I have a lot of duplicate items and other stuff I don't really need in my inventory. Is there any way I can sell all of this stuff and get some credits?
There is no direct way to turn most items into credits in Metroplexity. A very small number of items yield credits if you "use" them from your inventory. A few items you may run into early on include:
Probably the best way to get rid of unwanted items in a productive way is to join a gang (or start your own!) and donate them to your gang stash. If you are a member of a gang, you will have a "Gang Stash" tab added to your inventory. Going to that page will let you donate some or all of your items to the gang stash and convert them into "shipments" which can be used to purchase gang hideout upgrades or smuggler supplies.
Note that some seemingly useless items are components for various sorts of crafting. Also, after a few days you might be able to complete a quest and receive salvaging tools, which lets you break down many items into potentially useful component parts.
Finally, some items have quest-related uses. It is generally a good idea to keep at least one of each item you find, especially it if seems like it might be part of an outfit. For example, don't be too quick to get rid of every dirty old trenchcoat or pair of smelly old pants you get, even when you get your hands on better equipment — you can use them to disguise yourself as a bum later.
How do I get more credits?
If you have not yet finished the Docks Quest, finish that quest and take the 500 credit reward from Mikhail. If you have already spent those credits, or have declined the reward, credits can be difficult to come by early on.
Completing the Problems on the Docks quest can yield 400 credits if you got it from a security guard on The Docks. It can be a lengthy and at times difficult quest early on, though, so this is not a way to generate immediate cash.
See Credit Farming more full-spoiler details.
Should I join a gang?
There are many benefits to joining a gang. Probably the most immediate benefit is that if you are in an area where your gang has a Hideout (gangs can build one each at The Docks, Metroplex U, and Outside the Slags), you will recover more hit points while resting there and may get other benefits if various hideout upgrades are present. As described above, being in a gang also gives you a way to get rid of unwanted items by donating them to a gang stash. Many gangs have Smugglers, who can bring you luxury food, medical supplies, weapons, and other items. Being in a gang also makes it possible to participate in gang warfare, which is risky but potentially very rewarding.
In addition to the mechanical benefits, being in a gang gives you access to a private chat channel (type "/join gang" to listen to it) that you can use to coordinate gang warfare and discuss other gang matters. Your fellow gang members might be able to provide you with advice and help as you play through the game.
Most established gangs have fully-upgraded Hideouts. But you may want to strike out on your own and form your own gang. It only costs 250 credits to form a gang, and gradually building up your Hideouts and recruiting new members can be very satisfying.
How do I get more Energy?
You might find this page helpful.
Help! An NPC just died! Can I still finish all my quests?
Don't panic! Look here for comforting details.
I hear resetting makes you lose all of your skills and techniques. Why would I want to do it?
You can find more details about resetting here.
There are multiple paths through almost all of the content in Metroplexity; many of those paths feature mutually exclusive content. Resetting allows you to get items, and experience skills, contacts, quests, and other content that may not be available during your current run. If an important NPC died, resetting gives you another chance to try and save him or her. (For that matter, if an NPC did not die, you might be curious to see how the games changes if s/he does die…)
There is also the beta reset competition to determine who can play through the main quest line the quickest, and the longer term competition to see who can collect the most skills — both of which give unique rewards.