Here is some links will helps to create a multisite of drupal 7 

1. https://www.ostraining.com/blog/drupal/drupal-multi-sites/

2. https://www.drupal.org/docs/7/multisite-drupal/configuring-a-basic-multi-site-development-environment-in-linux

3. https://www.ibm.com/developerworks/library/wa-multisitedrupal/index.html

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.

Thanks