The tutorial videos below will help you add content to and edit your pages. Under the "Insert" tab on the right you will see that you can insert text, images, folders from Google Drive, Google documents (Docs), Google spreadsheets (Sheets), Google Maps, Google Slide Shows (Slides), and YouTube videos. You can also embed a VoiceThread presentation by copying the embed code from the Share menu on the VoiceThread file and using the "embed" insert in Google Sites.
If you embed content from a site like Google (Docs, Sheets, etc.), you will need to make the content public by sharing it with others. A "Share" button will appear in the upper right of the menu of the item you want to embed.