Creating a 2D object is the same as creating a 3D object. You just need more vertices you would go from 4 vertices in a square to 8 to create a cube. Also, the same happens to indices, vertex color and normals.
If you want to properly set up a 3D shape (a cube) with proper texture, you need to calculate the texture coordinates. The reason you don't do this yourself is because is inefficient. You need to use blender to get the proper coordinates for vertices, normals, colors and textures.
Using blender usually takes about 20 seconds to create a cube with textures or to create a 3D human character. If you do the first manually it would take you about two days to complete. More over, if you do a 3D human character manually it might result impossible due to the number of coordinates multiplied by 4 because of the vertices/normals/color/texture.
My suggestion is to use blender, you should watch a youtube tutorial. Then you would be ready to go in less that 30 minutes.