We are fellow users here on these forums, you're not talking to iTunes Support. The questions should only be asked on the first purchase on a new computer or device, but other people have also posted recently about them being asked when it isn't. If you are being asked them then you will need to answer them before you can purchase.
If you can't remember their answers and you have a rescue email address (which is not the same thing as an alternate email address) set up on your account then go to https://appleid.apple.com/ and click 'Manage your Apple ID' on the right-hand side of that page and log into your account. Then click on 'Password and Security' on the left-hand side of that page and on the right-hand side you should see an option to send security question reset info to your rescue email address.
If you don't have a rescue email address (you won't be able to add one until you can answer 2 of your questions) then you won't get the reset option - you will need to contact iTunes Support or Apple to get them reset.
e.g. you can try contacting iTunes Support : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Account Management , and then 'Forgotten Apple ID security questions'
or try ringing Apple in your country and ask to talk to the Accounts Security Team : http://support.apple.com/kb/HE57
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 this page to add a rescue email address for potential future use : http://support.apple.com/kb/HT5312 . Or if it's available in your country you could change to 2-step verification : http://support.apple.com/kb/HT5570