Sharepoint STSadm command to build a new site from a template

I am pretty new to sharepoint administration and i am trying to learn the STSadm command line. I would like to automate the creation of new sites. I have a test machine with sharepoint installed and i was trying to run the following command but it was failing.

stsadm -o createsite -url http://spsandbox/testsite/newtest -owneremail email@company.com -ownerlogin domain\myusername

When trying to run this i receive the following message

"Another site already exists at http://spsandbox. Delete this site before attempt
ing to create a new site with the same URL, choose a new URL, or create a new in
clusion at the path you originally specified."

Im not sure what is going on here as I would like to create a new site under the parent site but it looks like it wants to overwrite the parent site?

I have attached a screenshot of the main site page. I would think that creating a new site would just add it to the top navigation bar.
1.jpg