Cannot fill bezier curve using Ckey....

I’m new to blender and have been working my way through some tutorials but have hit a brick wall with Bezier curves. I can create and shape a curve fine but when I use the C key to close it the shape won’t fill. This is only a problem in 3D. It seems that the curve becomes twisted and I can’t seem to straighten it out. Can anyone help???
All posts very much appreciated.

Curves can be filled only if they are pure 2D. There is a button named “3D” which should be unchecked. The reasons are mathematical: the interior of 3D curve is not well-defined. If you want a 3D surface, you should select a Surface primitive for it. Or you can model your curve in 2D, convert it to a mesh, and additionally add a subsurf modifier to it. For surface-type of models it is usually preferred to go with subsurfed meshes only.