plugin functions and Ajax help

i am trying to write a plugin that processes ajax post requests makes a database request and returns json as the result. right now i am just trying to get the request and response right. the problem that i am running into is that the parse_request function (rmm_add_signature()) is being run on every page load on my site. and when i make an ajax call it returns the entire page.

my question is, how do i only run that function when it is called via the url, and secondly how do i keep the rest of the theme from loading as well?

function rmm_add_signature()
{
	$fields['pet_id'] = $_POST['pet_id'];
	$fields['F_Name'] = $_POST['F_Name'];
	$fields['L_Name'] = $_POST['L_Name'];
	$fields['Zip_Code'] = $_POST['Zip_Code'];
	$fields['Email'] = $_POST['Email'];
	$fields['Comment'] = $_POST['Comment'];
	$fields['Opt_In'] = $_POST['Opt_In'];
	//wp_die('');
	echo json_encode($fields);
}
add_action('parse_sig','rmm_add_signature');
function rmm_query_vars($vars)
{
	$vars[] = 'my-plugin';
	return $vars;
}
add_filter('query_vars','rmm_query_vars');
add_action('parse_request','rmm_add_signature');

this is my first crack at writing a wordpress plugin. however i have been working with php for the last year or so.