Talk about issues concerning your school. Actual issues. Say why it is important to you and how you can make things better and jazz it all up. Find out from classmates what they really think the issues is. The point is to get people to listen to YOU and YOU only. You've got to be the center of attention when you're talking. Don't just read from the paper. Eye contact with many of who you're reading it to. It shows your dedicated and willing to go far.
Tell your strengths. "I'd be good as Vice President of Student Council because...(example) I'm a great leader, I have really great characteristics for it. I listen to the issues that my peers think should be brought up to the school. I want to make a difference so that I can make school a more enjoyable place."
Don't put ANYTHING down or bad about you in your speech. AT ALL. If you had something like "I'm a good worker but I tend not to listen well" then, I'm sorry, but you are out. Instead, say "I'm a good worker." It shows up a lot better.
Now, I know you asked for a main topic, so find out what the issues are in your school that are important to your peers AND some teachers. Once you have that, you can go straight from there. If you are really desperate and in need for a topic, write about this: "How I can make our school a better place and more enjoyable for everyone." Just go from there, and good luck