3. Putting polys in your thingy
Make sure your cube is still selected, with the 'Select Object' tool in the main toolbar selected. If your cube is selected it should be highlighted in the top/side views and have a sort of border in perspective view, like in the picture. Now on the Command bar to the right, click on the Modify tab (blue half arc). Give your cube some segs like I have by typing in a number in the command bar, or using the up/down buttons. You should see your cube being divided up in your non-perspective views.
You should know that any 3D model is made of triangles. Trangles make squares, squares make cubes and so forth. Keep this in mind when making your model. These triangles are called polys, the more polys in your model the slower it takes your computer to "draw" them. The sage engine in Generals is good at handling polys, but that doesn't mean you can go and make 10000 poly tanks. Remember that you may be making lots of these tanks in the game. A good target number of polys overall for Generals is about 200-500. For Renegade it's about 2000-3000, as not so many tanks can be used at one time in Renegade. You can check how many polys are in your model at any time by clicking the utilities tab on the command bar (looks like a hammer) and clicking the 'Polygon Counter' button.
Once you've given your cube it's segs, right click your object and convert it to editable mesh.