Example child theme which allows you to add custom font from Google Fonts to your WordPress theme.

functions.php

<?php

   /**
    *
    * child theme functions and definitions
    *
    */

    add_action( 'wp_enqueue_scripts', 'child_theme_enqueue_scripts' );
    function child_theme_enqueue_scripts() {

        # enqueue google font with this code line
        wp_enqueue_style( 'child-theme-fonts', add_query_arg( 'family', 'Roboto', '//fonts.googleapis.com/css' ), array(), null );

        # enqueue parent theme style
        wp_enqueue_style( 'child-theme-parent-style', get_template_directory_uri() .'/style.css' );

        # enqueue child theme style
        wp_enqueue_style( 'child-theme-style', get_stylesheet_directory_uri() .'/style.css', array( 'child-theme-parent-style' ) );
    }

   /**
    *
    * end of file.
    *
    */

?>

style.css

/*
Theme Name: Child theme
Template: YOUR_ORIGINAL_THEME_NAME_HERE
*/

h1 {
    font-family: 'Roboto';
}