Assigning thickness to a 2d object in TC is not recommended. There are several tools to make 3d objects out of 2d objects, the extrude tools for example. Saying you want to create a solid is ambiguous. We think of solid 3d objects like cones, spheres, cubes, etc. A surface would be like a crumpled sheet of paper. 3 dimensions but no volume. Solid and surface have different meanings in TC. They describe the way the program deals with 3d geometry. The 3d tools are the best way to create 3d objects. In your drawing the lines should be extruded rather than assigned thickness. I'm sure some one will point you to some tutorials.