Create a document with all the styles you want. Leave the text blank (or at least, have it contain only the text you'll ALWAYS want).
Use the "Save as Template ..." command in the File menu. You'll have to give it a name like any other file, and it will be automatically saved in the Pages templates folder.
Go to Pages preferences. You can either have Pages always open that template, or have the Pages template chooser display every time you want a new document.
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