GreenGeeks offers WHM free with all reseller hosting plans. You can learn everything there is to know about about WHM in the official documentation.
We also have «getting started» articles that will take you through the process of creating hosting packages and user accounts step by step:
Reseller Account Set up, Step 1: Creating Packages
Reseller Account Set up, Step 2: Creating Accounts
Note that some features in the official WHM documentation may not be available as they are only available to GreenGeeks administrators.