Go to World (at the top left) > Cells. In the list of Cells, select yours and go to the Interior Data tab. You can set ownership of the cell there, which affects everything in it that isn't specifically assigned to an NPC.
For specific items, load the cell in the Render Window and double click the items you wish to change to bring up it's Reference pane. Look for an Ownership tab, as seen here:
Assign Player or PlayerFaction as the owner and voila, all yours.