Take a look at the WooCommerce docs here: Tutorial – Customising checkout fields using actions and filters
your code would look something like this:
function my_custom_checkout_field( $checkout ) {
woocommerce_form_field( 'my_field_name', array(
'type' => 'select',
'class' => array('my-field-class form-row-wide'),
'label' => __('Question?'),
'placeholder' => __('Enter something'),
'options' => array(
'Yes' => __('Yes', 'woocommerce' ),
'No' => __('No', 'woocommerce' )
)
), $checkout->get_value( 'my_field_name' ));
}
though the input also needs processing to be displayed in the WC admin. If you have little experience with PHP, it’s not that easy though. You could take a look at this plugin for inspiration.