You can open the Word Options Box by right clicking the ribbon.  Choose the option called “Customize the Ribbon…”. (You can also access it through clicking File, choosing Options, and then choose Customize the Ribbon.)

The customize the ribbon contextual menu.

The following dialog box will appear.

The left hand side contains a box that contains a list of tools or commands.  The drop down box near the top will give you different sets of commands to browse.  By default it lists the “Popular Commands”.  I prefer to set this to “All Commands” most of the time when I am customizing the ribbon.

The right hand box contains a list of the tabs available.  You can click the boxed plus symbols to expand the tabs to see the groups inside.