Go to Fort Independence and talk to a Brotherhood Outcast named Morgan.

She is underneath a bridge with a turret you will know when you find her because she tells you to keep your head down.

There will be a couple of raiders.

Kill them both and talk to Morgan ask every question, that's what I did.

Become the scavenger find them laser pistols, power armor, etc.

Eventually you will gain access to go through the gate there will be a guy named Protector Casdin.

Talk to him and give him the stuff you have found, note he does not accept assault rifles, shotguns, conducts, stuff like that; only energy weapons, power armour,and the alien blaster if you have it.

You have choices of either .556 ammo, frag grenades, or stim packs.

It depends on what type of weapon you have. The better the weapon the better the reward. I have 95 stim packs.