I've forgotten the answers of the questions for the apple ID ,so i can not buy anything now , what can i do ?

If you have a rescue email address set up on your account then you can try going 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 might see an option to send security question reset info to your rescue email address.

If you don't have a rescue email address set up then go to Express Lane and select 'iTunes' from the list of 'products' in the middle of the screen.

Then select 'iTunes Store', and on the next screen select 'Account Management'

Next choose 'iTunes Store Account Questions' or 'iTunes Store account security' (it appears to vary by country) and fill in that you'd like your security questions/answers reset.

You should get an email reply within about 24 hours (and check your Spam folder as well as your Inbox).

Or you could see if the second half of Kappy's reply in this thread helps : https://discussions.apple.com/thread/4530659