There’s little time to make sense of your predicament. Follow the corridors of the vault to the overseer’s terminal and open the evacuation tunnel.
Head east and discover the settlement of Sanctuary. Locate Codsworth outside your old home and explore Sanctuary with him (or clear the area beforehand). Watch out for the poisonous spit of any Bloatflies you encounter.
Codsworth recommends widening the search for Shaun. Head southeast; you’re likely to run into a friendly dog at the Red Rocket Truck Stop. He’s sure to be a faithful companion. Perhaps the first of many.
Companion: Dogmeat
The pond northwest of Concord is a breeding ground for Bloodbugs—a disgusting winged fly with filthy proboscis strikes. But the real threat are the Raiders holed up in the town. This foul-mouthed bunch of psychotic wastrels favors any sort of weapon that mangles. They cannot be reasoned with.
Search the Museum of Freedom here to encounter Preston Garvey of the Minutemen. Consider helping his faction before moving on.
Otherwise, your exploration toward Boston continues.
Companion: Dogmeat