Hello there, we are glad to have you here!
If you are new to the subject, then we recommend you start with the little Introduction into Mutation Testing. Then, install Mull and go through the tutorials.
As soon as you are comfortable with the basics you may want to learn about various options and settings Mull has, as well as pick the right set of available mutation operators.
Doing research? There is a special chapter for you: For Researchers.
If you want to dive deeper and look behind the curtains, then we encourage you to hack on Mull.
If you have any questions feel free to open an issue or join the great community of researchers and practitioners on Slack.