Go to www.google.com/fonts and find the required font family. Select 'Quick use' (icon highlighted in the image below).
In the next screen you can select styles and character sets you want to be available to your website,
You then have the option of importing the font into your website in three different ways. You can either use the standard link method, @import or Javascript. Add the relevant code snippet into the head section of your html, using the DocumentHeader.aspx page control:
The css can then be added wherever it's required in the styles. For example, to change the font across the whole site, add the following to lib.generics.css in the master style:
body { font-family: 'Open Sans', sans-serif, verdana; }
Other font libraries can be used in the same way. Essentially you just need to generate the link on the font website, add that to DocumentHeader.ascx, then specify the css as required.