Firstly you will need to sign up for a Google API key for your website: http://code.google.com/apis/ajaxsearch/signup.html

Once registered, put the following in the header section of your web pages replacing the key parameter with you registered key.

<script type="text/javascript" src="http://www.google.com/jsapi?key=ABCDEFG"></script>

To use Google’s libraries use the google.load() function:
For jquery:

<script type="text/javascript">
   google.load("jquery", "1.3.2");
</script>

For MooTools (replace 1.2.4 with the version that is required):

<script type="text/javascript">
   google.load("mootools", "1.2.4");
</script>

Alternatively, you can also have an external link to the library:

For jquery:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>

For MooTools:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>

For a complete list of option, please see the Developer’s Guide http://code.google.com/apis/ajaxlibs/documentation/index.html#googleDotLoad