To add STP template for a new SharePoint site collection you will need to take the following steps in command line using the stsadm.exe command. These type of tempales are called ‘Server Admin Tempalates’.
The command is as follows. Note that the description is optional.
stsadm -o addtemplate -filename <template filename> -title <template title> [-description <template description>]
You will be prompted to do an iisreset to complete the template installation. Once iisreset is complete login to a web site create a new site and under “Custom” tab for “Template Selection” you should see your new template as an option. You can find greate Microsoft tempales from here Application Templates for Windows SharePoint Services 3.0.
Once this is completed the template will be avaliable from the SharePoint Central Admin or the self-service creation if avaliable. If you need to add a STP template to an existing SharePoint site try this entry I wrote up at How to add a STP template to an existing SharePoint site.