The prompt to fill in your security questions does not appear immediately after creating your account. It usually takes a couple of days (or logins) for the prompt to appear, so there is no need to worry, you will get the chance to do it!
We advise you to fill in the answers with things that are easy to remember and that make sense to you. The security questions are quite important when it comes to account ownership verification, so do make sure that you know what the answers are.