What should you write about?

Answer 1

Write about whatever is interesting enough to make you want to write about it! If you try to write about what other people think is interesting, or write what you think is going to sell a lot of books, you're going to end up bored and write a dull book (if you even finish the thing at all). If you pick a topic that is interesting to you, then you will be motivated to do the research necessary to write a good book, and will want to put in the hours of writing and editing that you will need.
If you are asking about writing a shorter work, such as an essay or short story, the same idea is true - write what is interesting to you. If the teacher gives you a topic, pick some aspect of that topic that makes you curious enough to go look up some information on Google.com or dogpile.com. If you are trying to choose a topic, pick one that relates to your own interests and hobbies.
The phrase "write what you know" is a good guideline. This does not mean that you cannot write anything that you have not experienced yourself - if you are interested enough to research a topic and learn all about it, then you do know that topic!

Answer 2

You should write about topics that you are passionate about or have expertise in. Choose a subject that interests you and that you believe will capture the interest of your audience. Conducting research and organizing your thoughts before writing can also help you develop a strong and engaging piece.