You can write articles on almost any topic. Exceptions are mostly obscene topics, or articles with a heavy bias (especially for subjective matters). I suggest beginning with topics that you know about, to keep research at a minimum, such as hobbies. If you enjoy knitting, write an article about what materials you need to start knitting. If you enjoy Italian food, write an article about what makes an Italian dish stand out. These articles may or may not sell, but it's the best way to get a feel for CC's submission guidelines.
Please be sure to review your article before submitting it. Spelling mistakes, incomplete sentences and punctuation are the fastest way to have your article rejected.