Work always with Paragraph and Character Style. Not even one single letter should be without a Paragraph Style. (I am not recommending to use the the Default style in brackets!)
Create a text variable based on your headline or any text you want to appear in the header.
Make a text frame on the master and insert the text variable. Take care that this text frame (as well as those for page numbers) is outside the margin, otherwise it would overlap with the the content of the page. Put the header (and the page number) frames on its own layer on top of any other layer. Align the text frame with the margin, so you can later apply different page settings and use layout adjustment.