It should interesting and significant to the readers.
Pick a specific topic. If you want to talk about Music, pick something a bit more specific like a band or a genre.
You can get ideas from school events.
Have different editorials about different things. You can't have two editorials that have the same topic!
Talk to other people about what to write. They could give ideas
Research a lot about your topic so you can have an enough information to give your opinion without being very biased.
Try not to write about an issue that has been talked about a lot UNLESS you have new information or give a new point of view on the issue. :)