Just find a merchant.
Moira in Craterside Supply in Megaton, Flak or Sharpnel in Rivet City Market or so on.
Talk to them and select the dialogue option that says "Let me see what you have for sale."
On the left is your inventory. And the right is the merchants.
Select some item in your inventory and choose it. It will appear in the right screen as [_] *item name*
And you should see a arrow that says *amount* caps, pointing in your inventory.
Choose the accept button in the bottom of the screen and voila, you sold something.
Was it that hard to figure out?