I want to put a header in my school paper, and my teacher requires 1" margins. I had the margins set to 1", but adding the header made it much bigger. I can try to estimate one inch, but it isn’t easy. Is there a way that I can make the header fit inside the margin space?
There is a conflict hidden in your request. The margin is the area round the page where nothing will be printed and you want to print in it 
What I think you want to do is to set the top margin to half and inch, then create a header which is half and inch and put your heading in there (adjust the sizes to suit, provided that they add up to the required inch).
First, go to Format → Page → Page and set the top margin to 0.5 inches.
Then go to Format → Page → Header. Make sure that “autofit height” is not ticked, then enter 0.5 inch in the height.
Now, from the edge of the paper working down, you have half an inch of true, unprinted margin fillowed by a half-inch header, bollowed by the bulk of your document.
Thanks a ton!
Note that there is one more setting on the Header tab, that adds to that total height above the text: namely, the header spacing is between the header height and the text below.
More details at FAQ.