First, on a blank document, change the font to Times.
Make any other changes to the document that you would like to have as default (margins, header/footer, page #'s, etc.).
Then File->Save as Template. Save it.
If you have Pages to open to the template chooser, just choose the template you just saved. If not...
Do Pages->Preferences. Choose For New Documents: "Use Template: ____" and click "Choose" and choose your template. Then every time you open Pages, the default document will have everything set however you like it.