How to create a top level site collection in SharePoint using PowerShell?
To add new site collection use the following cmdlets
New-SPSite –url <URL> -name <site collection name> -owneralias <site collection admin>
-template <template name>
Sample PowerShell to create new site collection in SharePoint 2010
New-SPSite –url http://virtualizesharepoint -name Home -owneralias vSharePoint\SpAdmin
-template STS#0
If you want to know the list of templates : http://virtualizesharepoint.com/2012/01/03/site-template-names-and-ids-in-sharepoint-2010/
Aryan Nava 