How to make search for the custom post type?

A better approach is to address the main search query. @Dipesh’s answer is technically correct, but I would suggest using the pre_get_posts action instead to affect the intial query, rather than generating a new one. Much less work!

add_action( 'pre_get_posts', 'se39294_search_pre_get_posts' );

function se39294_search_pre_get_posts( $query ) {
    if ( $query->is_main_query() && is_search() ) {
        $query->set( 'post_type', array( 'page', 'post', 'questions' ) );
    }
}