Check out this page, and give em' hell for poor Charon.

fallout.wikia.com/wiki/Fort_Bannister

That link will for sure suffice for information.
In short, head West from Megaton, or if you have already found the Jury Street Metro station you can just walk west from there. It is northwest(ish) but there is really nothing else around so it's easy to spot.
The base is pretty crazy, I walked in on accident and spent a considerable amount of time pinned behind a wall because they had rocket launchers and crazy amounts of robo co technology there to stop you from getting into the base that is totally worth pilfering if you survive that place.
If you have bad Karma already my suggestion would be to get Jericho so that way they have a second target to chase around and it'll give you breathing room while you pick everyone off.

There in Fort Bannister it's H4 on the map in the URL below, but be careful there is a lot of them and they're armed to the teeth so go for a night attack.

fallout.wikia.com/wiki/Fallout_3_map

if you have done the quest involving the Family(Blood Ties) You should ask Karen Schenzy from Arefu for help and she will give you the location.. THere will be outer camps though