When working in a document, you should spend the majority of your time working on the content. When the content is complete, go back and decide the font, font size, font color, and any other styles you want apply. If there are headings in your document, you may want them to be a little larger than the rest of the text or you may want to make them bold. Do this on the first heading, but then you can use the Format Painter to copy that style onto the other headings in your document or even into a different document.
This video demonstrates how the Format Painter works.