First, close the vault door. Radroaches are a pain. Eat everything, not so good to eat. Don't want any deathclaws getting in either. They are just bad news all around in enclosed spaces.
Second. Since, of course, I left with a gun that the overseer gave me, I find the closest group of cannibal raiders, shoot them and take their guns and ammo. Befriend the dog that they didn't eat (they don't like dogs, just other humans, too chewy). Have the dog find dog food, makes a great stew.
Third. Find the Charles river, it's probably actually a clean river after humans have been underground for decades. May be a little radiated, but that just adds to the spice of fresh water.
Fourth. Make a shelter or find something that works. Only got a few weeks before it snows in Boston. Something warm and cozy. Boston Museum of Science probably is still standing. Might find some good stuff in there.
Next, start eeking out an existence.
Lastly, figure out how to get Tinder working in the Boston wasteland. Looking for a date.