From your wording, I'll guess you have little experience in working with wood. Here are some alternatives:
1. Have a local cabinetmaker do it for you. They can make a plywood cage and doors for a reasonable price.
2. Look at www.herpcages.com and www.precisioncaging.com. I have the 36x12x12 model from herpcages.com (he doesn't show a pic of that model on his site but does have it available) and it is a nice, clean-looking and functional cage for kingsnakes like yours. The Precision 24x20x13.5 is also a great cage for kings.
3. Look at www.neodeshacages.com - the 3ft standard model is fantastic.
These are just some suggestions. If you'd rather make your own, by all means do it. Plywood works well and is much lighter than melamine, but you will need to paint it. I like doors that are hinged on the bottom (or sides) and swing down (or out) rather than sliding doors. I don't know how to tell you how to build a cage over the internet. Trial and error works the best. The basic tools you should have are a circular saw, a straight-edge, drill, router and sander. These tools are affordable and make things much easier. Good luck.