Log in using your Google account.
Choose Edit Page then try entering some text and formatting it e.g. Indent, add bullets, make bold etc.
Practice using Link (from the menu bar at the top of this page) to add a link to another web site.
Practice inserting adverts by clicking on Insert (from the menu bar at the top of this page) then choosing AdSense.
For additional help on how to do things, visit Google Sites Support.
PS - Don't forget to click Save before leaving the page if you want to retain you changes!