How do I join several objects into one? I'm creating side skirts for a truck and I am duplicating the frames I want. The problem is it's making the duplicated object into another object. How do I put it into the same one?
You can join objects by selecting them (ShiftRMB) and pressing CtrlJ in the 3D view while on Object mode.

The selected object(s) will be joined to the active object.

The active object the one with the yellow outline, usually the last object selected.