first create a user from this function
$user_id = wpmu_create_user( $username, $password, $email );
then used this function to create blog
wpmu_create_blog( $newdomain, $path, $title, $user_id , array( 'public' => 1 ), $current_site->id );
for detail you can see this file
/wp-admin/network/site-new.php
after viewing this page you will have the exact idea what you want to do.