Use wp_enqueue_style() in your functions.php:
function custom_wp_enqueue_style() {
wp_enqueue_style( 'bootstrapcdn', "https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.5/css/bootstrap.min.css" );
}
add_action ( 'wp_enqueue_scripts', 'custom_wp_enqueue_style' );
The second parameter can be the path to the folder in your server where you have your bootstrap.