There are instructions on this page for how to create a new account without giving a credit card : http://support.apple.com/kb/HT2534 i.e. select a free app and click 'create Apple ID' when 'buying' it.
For Siri yes, with iOS 6 you should get it on an iPad 3 (iOS 6 will remove the built-in YouTube app and the Maps app will change).
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