I just wanted to draw but I can't create new file because the 'Create' button is grayed out :c

http://i.imgur.com/VCvKuRL.png

That's because you're in the create-from-clipboard page and there's nothing on the clipboard.

I am somewhat experienced user of Krita, and I have same confusion everyday in this dialog window

Set preferred dimension, try to click "Create", change "clipobard" to "custom document" (and dimensions are lost)