Special Note about the fat you get:
Any animal fat (cow, deer, sheep, buffalo) can be rendered into tallow, but the quality of the fat you use will determine the quality of the tallow. Many people swear by only using "kidney suet", the fat that surrounds the cow's kidneys. It's much harder and whiter and makes really wonderful tallow. It's also really hard to find anywhere but a specialty butcher shop - and you're likely to pay a pretty penny for it - whereas normal, everyday beef fat from the grocery store butcher is likely to be free. Is there a difference? Probably - to the purists at least - but I haven't noticed a big difference in the soaps I've made.
As you'll see from looking at commercial soap labels now, tallow is a common oil that folks use to create soap recipes. Whether you choose to use animal oils or not in your soap recipes, understanding the qualities of soap making oils, and at least knowing how to render your own tallow is a soap making experience that I think all soap makers should have at least once.