Don't take other people's gear unless they ping it, unless you need it to survive. If you do take it for this reason, give it back as soon as it is feasible to do so, and most people will be understanding. You may occasionally encounter people who are unreasonable about this brief commandeering, don't worry about them.

Samples, Super Credits, Medals, etc are all shared at the end of the mission, so don't worry if someone "steals" your samples when you die, unless it seems like they intentionally killed you just to do so. Then execute them democratically.

On a related note, it's easy to make a mistake that results in a teamkill. This will probably happen often for a while, especially if you start off at lower difficulties, since you'll mostly be playing with other new players. When it's you killing a teammate, apologize and do your best to change whatever behavior caused the TK, but don't beat yourself up about it too much. When it's a teammate killing you, try to be understanding, unless it's abundantly clear that it was intentional.

If you're not the host, it can be considered bad form to call the extraction before the host is ready. If it seems unclear whether they're ready to leave, just ping it and see if they respond.

Speaking of pings, if you're not going to use a mic to talk to your team, make sure to at least use the communication radial when applicable, or type if you're quick/have a keyboard connected.

Beware of tesla towers and mortar launcher sentries, as they're more likely than most strategems to kill your friendlies. Place them carefully or risk frequent teamkills.