If you choose to write daily, you can write about a whole number of things. You can write about anything from your goals for your day, any questions you have and want to research (about anything at all), things you feel you want to improve on, to something more trivial like how you feel about the weather. What you write about isn't as important as how you write it. Paying attention to how you write, what words you use, and writing techniques you employ is much more important.