How to fill in surfaces

How can I make wireframe box red?

Is it a solid? Switch to a shaded view mode.

Well it was a curve that I extruded up. Now I it has polysurfaces that I can select

Okay so it’s at least a polysurface then, unless there’s some error that it’s rendering ‘inside out.’ There are a couple different places to set the colours of objects. Select it, go to the Properties tab, and the first place you’ll find a setting is under “Display color,” which is the colour of the wireframe and surfaces in “Shaded” mode. The other place is found by clicking on the little paint-tube icon in the properties panel, which takes you to the “material” which is for rendering and the rendered view mode.