Killing stuff, however early in the game this is not an easy task due to lack of ammo. Just sneak into all the house of Megaton and steal every item, yes every item, and sell it to Moira. While 0.9 caps may seem insignificant, using this method can get you enough caps to buy ammo which in turn provides a way to kill things. If you have enough ammo, just start killing Enclave soldiers and looting their Power Armor and selling it to the traders at Canterbury Commons. Also scrap metal, sugar bombs, and Pre-War books should be hoarded so they can be traded in for large amounts of caps at their quest locations. Traveling to Minefield and going into each house can provide a decent amount of Pre-War books which can be turned in at the Arlington Library for 100 caps each plus experience.
Be Very Evil.
In Evergreen mills (If you can manage to get past the Raiders) In the bottom of the Bazaar is Smiling Jack, simply killing him will give a nice amount of caps. (He had 1300 on him when I did it, along with a lot of weapons and ammo that can be taken and sold elsewhere.) And you can grab a nice Shotgun off his corpse and a Bobblehead behind him. If you want the max amount of caps have him repair as much of your stuff as possible, or even buy weapons off him then have him repair them, then kill him and get all your caps back, and then go sell the newly repaired and improved in value weapons.
Have a high repair skill. Salvage armors/weapons from raiders/enemies you killed and repair them. Repaired items have higher value. Also, from salvaging and repairing each you will not clutter your inventory. Now I am very reach! I can buy Charon (I'm a Very Good Person) early in the game!