If you want to set your template for an existing site or subsite, go to the sitemap editor, choose the "Resources" view and open the properties dialog by clicking on the burger menu of the main folder of the (sub-)site. Then, in the tab "Complete Properties", set the property template to your
template by choosing it (via its name "My Template") from the drop-down list.
If you want to create a new site that uses your template, use the "Site Management" in the administration view. Choose "New site" and fill out the form, in particular "Title", "Folder name", "Server URL" and "Template". The remaining things can be empty. For "Template" enter
template=/system/modules/my.template/templates/main.jsp. For "Server URL" you may choose "http://localhost:8081". If you created the site, go to the explorer view and switch to the newly created site. In this site's root folder add an
index.html by choosing "New" (see the wand icon at the upper left corner of the explorer view) and selecting "Container page" in the appearing dialog. Click "Continue" and set the file name to "index.html".
In order to check if everything works fine, go to the
index.html directly in the main folder of your site or subsite. You should see your template JSP coming up and the toolbar of the page editor should appear at the top.