Here is a great tutorial on how to draw a 3D cross that is easy to complete and looks great. The techniques that you learn making this drawing will help you make more complicated 3D structures in other drawings you might want to make. If you have the time then we can do it right now, ok?
First place a cross approximately where you want the 3D cross to be then use straight lines to make the cross and front faces that give it the 3D look. We then add the colors as you can see in the how to draw a 3D Cross tutorial to give a wood-like look. To give the finished picture more appeal we fill the background with a smoky/cloudy style.