How to add custom field to custom post type?

As Muhammad Yasin said there are plugins I'd recommend:
http://wordpress.org/extend/plugins/more-fields/

if you want to do it yourself in code look at: add_meta_box

<?php add_meta_box( $id, $title, $callback, $post_type, $context, $priority, $callback_args ); ?>

You can register boxes per post type.

If you want to create a custom meta box inside a custom post type you will need to use 3 functions.

1. A function to create a custom "meta boxes" block/screen on your post edit screen: add_meta_boxes_{$post_type}

2. A function to add a input field to change/show your custom meta

3. And finally a function to save your meta along with the rest of the post when you click save on the post edit screen: save_post_{$post->post_type}

In your case, a custom checkbox would look like this:

add_action( 'add_meta_boxes_products', 'meta_box_for_products' );function meta_box_for_products( $post ){    add_meta_box( 'my_meta_box_custom_id', __( 'Additional info', 'textdomain' ), 'my_custom_meta_box_html_output', 'products', 'normal', 'low' );}function my_custom_meta_box_html_output( $post ) {    wp_nonce_field( basename( __FILE__ ), 'my_custom_meta_box_nonce' ); //used later for security    echo '<p><input type="checkbox" name="is_this_featured" value="checked" '.get_post_meta($post->ID, 'team_member_title', true).'/><label for="is_this_featured">'.__('Featured Product?', 'textdomain').'</label></p>';}add_action( 'save_post_team_member', 'team_member_save_meta_boxes_data', 10, 2 );function team_member_save_meta_boxes_data( $post_id ){    // check for nonce to top xss    if ( !isset( $_POST['my_custom_meta_box_nonce'] ) || !wp_verify_nonce( $_POST['my_custom_meta_box_nonce'], basename( __FILE__ ) ) ){        return;    }    // check for correct user capabilities - stop internal xss from customers    if ( ! current_user_can( 'edit_post', $post_id ) ){        return;    }    // update fields    if ( isset( $_REQUEST['is_this_featured'] ) ) {        update_post_meta( $post_id, 'is_this_featured', sanitize_text_field( $_POST['is_this_featured'] ) );    }}

You can use this plugin

http://wordpress.org/extend/plugins/types/

or this tutorial may helpful for you.

http://wptheming.com/2010/08/custom-metabox-for-post-type/