Before you get into making your own lye, do some reading up on the science of saponification. One of the reasons lye soap gets a bashing (and often well deserved) is that without some seriously decent chemistry lab equipment, you cannot measure the strength of your home made caustic. Not enough fat to the batch and you can burn yourself badly.
Make your first batch using store bought lye. A five dollar bill will garner enough lye to make enough soap to last a couple decades, even if you are a slob like me.
Also find your way onto several soap making forums, they are pretty helpful folks in those places. And google "lye calculator".
