The most important thing when pairing fonts is contrast. You want to make sure that you can very clearly tell the difference between your headers and body text. Check out some of our favorite pairings of Google fonts.
Header: Playfair Display (Bold) https://fonts.google.com/specimen/Playfair+Display
Body: Source Sans Pro (Regular) https://fonts.google.com/specimen/Source+Sans+Pro
Header: PT Serif (Bold) https://fonts.google.com/specimen/PT+Serif
Body: Montserrat (Regular) https://fonts.google.com/specimen/Montserrat
Header: Poppins (Bold) https://fonts.google.com/specimen/Poppins
Body: Poppins (Light) https://fonts.google.com/specimen/Poppins
Header: Oswald (Medium) https://fonts.google.com/specimen/Oswald
Body: Halant (Regular) https://fonts.google.com/specimen/Halant
Header: Roboto Slab (Bold) https://fonts.google.com/specimen/Roboto+Slab
Body: Roboto (Regular) https://fonts.google.com/specimen/Roboto
Note: We recommend using Header fonts at 24-32px and body fonts at 14px, as a general rule. Of course, there are exceptions, but this is a great place to start.