My mum and step-father might be getting a divorce what am I supposed to do??

I'm sorry you may be going through this. You don't have to do anything. Know that their divorce has nothing to do with you, it does not change their feelings for you.
When you have questions ask them don't keep your feelings pent up.

I am sorry, first you understand it is not YOUR fault, and you stay out of the ways, don't get involved. This is their issue and you should just let them handle it