Multisite on CiviCRM with Drupal 7
Yes, You can use the same db of drupal as well as use the separate db, civicrm gives the option to add another db.
If you want to use another db for civicrm then you have to create the db first and enter the name at time of installation , If you are first time installing civicrm use this doc .Civicrm use civicrm.settings.php to use the database connection,you can also check out that.
And as you said you have multisite installation you can placed the civicrm at /sites/all/modules to be accessible to all your drupal sites