Here's a tiny XHTML example. Follow the link and then view the page source to see the DOCTYPE and html tags. To view the page source, go to your browser's menus and look for something like "View Source" or "Page Source". For Internet Explorer on Windows, for example, it's "Source" under the "View" menu.
Here's a good XHTML reference, with examples.
An example of acceptable image use.
Some PHP examples