Terms & Conditions click box upon checkout?

you can add a filter to your functions.php

add_filter('woocommerce_billing_fields', 'custom_woocommerce_billing_fields');

	function custom_woocommerce_billing_fields( $fields_array )
	{
		$fields_array['agb'] = array(
			'name'        =>'agb',
			'type'        => 'checkbox',
			'label'       => __('Mit Abgabe einer Bestellung best?tigen Sie, unsere AGB gelesen, verstanden und akzeptiert zu haben.', 'woothemes'),
			'placeholder' => __('AGB', 'woothemes'),
			'required'    => true,
			'class'       => array('notes')
		);

		return $fields_array;
	}

found here