The Complete Beginner's Completely Incomplete Guide to Oxygen Not Included
There are two things to look at: interests and traits.
Duplicants - aka dupes - will start with between one and three things they are interested in. The fewer the amount of interests, the more points they have in them.
The interests are things relevant to various tasks in the game: digging, building etc. You can choose an interest for a dupe to have using the dropdown menu next to the reroll button.
You can reroll dupes as often as you want. So if you want a good researcher you can roll and reroll until you get a dupe whose only interest is research.
There are no hard and fast rules here, so don't worry too much about dupe interests. But you will be doing a lot of researching and digging so a common approach is to have:
one digger (or digger/builder)
For your third dupe pick whatever catches your fancy. Some take a second digger. If I plan to do any ranching I often pick a rancher to ensure I get a good one (though the importance of this has decreased due to a long-standing bug having been fixed). Anything is fine, really.
Green traits are positive traits, red traits are negative.
There are traits in Spaced Out that aren't in the base game.
Some positive traits are more useful than others but none are game-changingly good. Do some rolling and rerolling and you'll get a feel for available traits.
The one positive trait I'll mention is Diver's Lungs. It means they use 25% less oxygen than normal. Which isn't a big difference, but can be useful when you're still learning the game. (But don't worry if none of your dupes have it.)
As far as what negative traits to avoid, this is also largely a matter of personal preference. I avoid:
dupes with Allergies as they spend half the day sneezing if you have certain kinds of flowers or plants in your base.
Anemic dupes, because they start out really slow.
Narcoleptics. Their naps aren't a big problem, more that they drop things when they fall asleep. A dupe carrying materials up a long ladder will drop the materials when a narcoleptic nap hits.
Flatulent dupes "produce" natural gas. Natural gas is convenient when you get it in large amounts from a geyser but inconvenient when you get it in small amounts inside your base.
Mouthbreathers. They use twice as much oxygen as standard dupes.
More dupes in-game
Every third in-game day you will get the option to add a new dupe to your colony. Surprisingly quickly you can even add more dupes than your colony can support - a fairly common "Ooops" for new players.
Add dupes sparingly while still learning the ropes: every new dupe is another mouth to feed and more oxygen consumed. (I have more on this topic in a later section: How many dupes "should" you have?)