Read the HT5312 page that you posted from, it has instructions for how to reset them i.e. if you have a rescue email address on your account then steps 1 to 5 half-way down that page should give you a reset link.
If you don't have a rescue email address then you will need to contact Support in your country to get the questions reset.
Contacting Apple about account security : http://support.apple.com/kb/HT5699
When they've been reset (and if you don't already have a rescue email address) you can then use the steps half-way down the HT5312 page that you posted from to add a rescue email address for potential future use