To start adding objects and apps to your site, just click Edit page, select Insert, and pick the object or app you'd like to insert.
When the embedded app (a Google Docs Presentation, for example) is updated at its source and published (in this example, within Google Docs), it gets updated automatically within Sites. Make sure you set the app to automatically republish changes, so your site's information is always updated.
Here are some of the objects you can insert:
An image: Click Insert, select Image, and either browse for an image on your machine or upload one from a specific web address. After you've uploaded and saved the image, you can click it to view it in full screen. You can choose one of these standard sizes for your image: small, medium, large, or original size. If you'd like to resize your image manually on the page, click the HTML button and within the source view, include attributes for width, height, or both, within the 'img src' tag.
A Google Docs Spreadsheet: Select Spreadsheet from the Insert drop-down menu, and a window appears with a list of the spreadsheets that you own or can view or edit in Google Docs. Click the spreadsheet you want to insert, or enter the URL in the box right below the list of spreadsheets. Click Select and you'll see the properties of the spreadsheet. Select your options and click Save.
A video: Click Insert, select Video, and upload a video from YouTube or Google Video. Paste the URL, enter a title, and save.
To easily embed rich media, retrieve feeds, or perform simple tasks like managing a to-do list, you can insert gadgets into your site. Click Edit page, select Insert, and choose one of the gadgets in the menu.
For example, insert the Recent posts gadget to display the announcements posted to the site, or the Recently updated files gadget, to show recent files. To view gadgets created by third-party sites, click More gadgets... and a window appears.
Select the type of gadget you want to insert by clicking the links on the left side, select a gadget, and click OK to save. If you'd like to first check how the gadget looks in your site, click Preview Gadget first.