Add logo in custom wordpress theme

The custom logo support should be added first to your theme using add_theme_support(), and then be called in your theme using the_custom_logo(). A custom logo is optional, but theme authors should use this function if they include a logo to their theme.

functions.php

function themename_custom_logo_setup() {
	$defaults = array(
		'height'               => 100,
		'width'                => 400,
		'flex-height'          => true,
		'flex-width'           => true,
		'header-text'          => array( 'site-title', 'site-description' ),
		'unlink-homepage-logo' => true, 
	);
	add_theme_support( 'custom-logo', $defaults );
}
add_action( 'after_setup_theme', 'themename_custom_logo_setup' );

OR

add_theme_support( 'custom-logo');

header.php

$custom_logo_id = get_theme_mod( 'custom_logo' );
$logo = wp_get_attachment_image_src( $custom_logo_id , 'full' );
if ( has_custom_logo() ) {
	echo '<img src="' . esc_url( $logo[0] ) . '" alt="' . get_bloginfo( 'name' ) . '">';
} else {
	echo '<a href="'.site_url().'"><h1>' . get_bloginfo('name') . '</h1></a>';
}