Rule Number One - this is the Zone, not the jungle.
If nothing else, keep that in mind. We are not going to fall to a kill or be killed, survival of the fittest mentality. There are still cities and towns out there, and fellow human beings in need. The country still exists to be rebuilt.
The boy scouts once believed you should leave the land as you left it before, if not better. Well. for now the land can take care of itself. Leave signs of where you stayed. You probably chose to sleep there because it was safe or well stocked. If others can find that area, they can benefit as well.
Note dangerous areas. Warn others.
Keep an eye on those aliens. Not all of them shout "destroy all humans" constantly - but there are plenty of unfriendly ones out there. If they're causing trouble, let others know and help put together a response.
Wave and call out. Let others know you're human. Fratricide is nothing but tragic.
Ruins are fair game, but stuff others are using is not. Sociopathy is repaid in kind - you steal and murder, others will hunt you down.
Stay a while. A lot of you are pretty anxious to roll into town, trade, restock, and then get back to exploring. You'll be a lot more welcome and get better prices in the future if you help out around the town. Repair the walls, plow the fields, mend clothes - whatever you can do to enrich the town will help.