If you want to create a button shortcode then just paste this code in your themes functions.php file.

//[button title="Visit Site" link="http://jasonellis.ca"]
function button_func( $atts ) {
	extract( shortcode_atts( array(
		'title' => 'Button',
		'link' => '#',
		'width' => '50',
		'window' => 'self',
	), $atts ) );
		
	return "<a style=\"width: {$width}px\" href=\"{$link}\" title=\"{$title}\" class=\"button\" target=\"_{$window}\">{$title}</a>";
	
}
add_shortcode( 'button', 'button_func' );

This will return a link called Visit Site that goes to the location specified. The width attribute (in pixels) gives the user control of the width of the button. The rest is pretty straightforward. You can add as many attributes as you need.