Where are the NPC settlements and vendors?
Here are some merchants you can sell stuff to:
Abernathy Farm - Connie
Drumline Diner - Trudy
Diamond City - Major hub/Many vendors
Good Neighbor - Minor hub/a few vendors
Grey Garden - Supervisor Green
Bunker Hill - Minor hub/a few vendors
Also vendors will visit your settlements (such as sanctuary) on occasion.
You will get a marker for Diamond City relatively early on during the quest:
When Freedom Calls