Be real. Even though it's tempting to emulate your favorites, it'll be hard to rap about your global cocaine empire if you're a teenager from the suburbs. You don't necessarily have to tell "the truth" 100% of the time, but you do have to be real and be believable.
Figure out what's unique about you, and what you bring to the table of rap. You don't need to have an ingenious or cookie-cutter answer to this question, but don't try to be just like another rapper, even if they're one of the best. In order to do well, you will need to introduce something fresh to the industry.