try to purchase app? but they ask me my security verification which i am not aware of. and i dont know the answer.

If 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 / Apple 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 this page to add a rescue email address for potential future use : http://support.apple.com/kb/HT5312

I know my security questions the message is asking for a security code.

Where are you seeing it ? Messages about security code usually refers to the 3 or 4 digit code on the credit card that you have on your account : http://support.apple.com/kb/HT3541