... or EN50522
For complex earthing systems you can use more advanced earthing software such as CDEGS, but there's not much point in doing that until you learn the basics, starting with reading the relevant standards and using the simplified equations from these standards. Start by reading the earthing standard (or standards) applicable to the project/country you are working on/in. IEEE80-2000 for North America and many other countries across the world, EN50522 for much of Europe and BS7430 in Britain.