Click "Manage My Account", sign in, and go to the Password and Security section. Answer the security questions and you'll be taken to a screen where you can then change them. If you've forgotten your answers, there should be a link just under the security questions fields where you can have a reset email sent to your Rescue email address. If the link for the email doesn't appear, as can happen if you didn't set a rescue email address or (apparently) have a .Mac email address, go here:
to report the issue to the iTunes Store.
Forum Tip: Since you're new here, you've probably not discovered the Search feature available on every Communities page, but next time, it might save you time (and everyone else from having to answer the same question multiple times) if you search a couple of ways for a topic, both in the relevant forums and in the Apple Knowledge Base, before you post a question.