Here is some links will helps to create a multisite of drupal 7
step 1 : create subdomain and locate path to main domain site folder.
step 2 : create db and db user
step 3 : create a new folder in sites folder with the any name , what you want .
step 4 : copy default.setting.php file from default folder to inside this newly created folder.
step 5 : rename the default.setting.php file to setting.php of newly created folder.
step 6 : open sites.php file and add the subdomain link correspondent to folder name.
for example :
$sites = array( 'xx.key2goal.com' => 'default', 'yy.key2goal.com' => 'yy_folder', 'zz.key2goal.com' => 'zz_folder', );
step 7 : now open url in new tab and execute drupal setting add db name
step 8: Finaly your multisite setting is completed.