Most AJAX will be triggered by one of 2 events, on page load or when a user clicks an object on your site.
1. The on page load event should be used only to prepare the site, formatting items for example but never use it for loading textural content that you want search engines to read.
We will now dig deeper into #2 as thats the most important. Search engines read links, if they cannot follow them they cannot read them. A link should look like this:
<a href="/my-great-content.html">My great content</a>
<head> ... <script src="//ajax.googleapis.com/ajax/libs/jqueryui/1.10.3/jquery-ui.min.js"></script> <script src="/ajax.js"></script> </head>
Inside ajax.js we can start our coding. Always check that the page is loaded before attaching any events to elements in the page, in modern browsers external libraries are loaded asynchronously and may load and be executed before your element is which will cause an error on occasion.
One important thing to note here is that we re-used the href URL attribute of the link using this.href, therefore we do not 'hard code' any URL references in our JS and it is more re-usable. Also any querystring parameters can be parsed as an object as the 2nd $.get parameter. More detailed information on AJAX requests can be found here.
Now your content will load even if JS is not enabled (you can do this in Firefox also to test, and is always good practise to confirm your site is usable without JS).
I would recommend AJAX development with the Firefox web browser and firebug to see what the AJAX is really doing and it allows you to inspect the content. All new browsers have their own web inspectors which can also do this but some can be rather difficult to navigate.
Good luck and happy AJAX coding!