A family of Microsoft word processing software products for creating web, email, and print documents.
If you are using the built-in header and page number formats, adding one format may remove the one you already inserted, as you have noticed.
What you can do is add the header first and then click inside the header and add a page number field directly in the text by pressing Alt+Shift+P. First you need to activate the header/footer view by double-clicking the header in your document.
For the details on creating cover pages, front matters, etc., see the link that Doug posted.