The best way to learn is to first build a wood rolling stock kit. You will learn construction skills and also have an idea how to design your own. Many kits are available at train shows and on the big auction site from Mainline Models, Ambroid, Quality Craft, Walthers, All Nation and others. Here are a couple of kits under construction.
Here is a brief tutorial and a scratch building thread from MRH I found online.
Bob