Freewriting: Write about anything that comes to mind without editing for a consistent period of time. Just keep writing/typing.
Clustering: for visual learners. Create a central topic in the middle of the paper. Connect subtopics to the central topic to create a web of ideas.
Journaling: Take a personal approach. Describe your reaction to what's around you, how you feel about a situation, what you like or what you hate.
Try one of these categories for topics!
If you're having trouble, maybe start with a cool character and write about what their life would be like.