Sounds like you are working with surfaces.
Try working in 3D using solid primitives from the solids toolbar. Right-click over any toolbar and click on Solids to get the Solids toolbar to appear on screen.
Read up the HELP files relating to the various tools on this toolbar and play about so that you can see the effects of the various tools. Read what is going on in the command line as you normally have to input sizes etc. as the commands are in progress.
You may also want the Solids Editing toolbar on screen at the same time. Same thing here again - read up the HELP files relating to the various solid editing tools.
Enjoy - it's hard work but a pleasure to see things created in 3D.