Welcome to Moodle.org Amjad.
Check out the documentation: http://docs.moodle.org/26/en/Upload_users#Deleting_accounts
Just make sure you don't delete yourself.
Just use bulk user actions (Users-->Accounts-->Bulk User Actions, add all users to the "Selected" users panel, and then select "Delete" from the list of actions to take. It will not delete site administrators, but after selecting everyone, then you should unselect anyone you don't want to delete.