Hi

Go to the page you want to create a sub-page under.  Select the "Edit this page" option.

On the edit page you should see the "Site Map" on the lefthand side.  You should also see 5 tabs: Index, Edit, Layout, Relations, & Publishing, with the page open on the "Edit" tab.  Immediately above the 5 tabs there are 7 small buttons starting with "Preview" on the left and ending with "Create Item" on the right (below the publishing tab).  Click Create Item and then create a new page (which will be a sub-page to the one you are on).  

You may not be aware, by there is a test site or "Sandpit" for administrators to play in.  If you add :4430 after you url (e.g. sitename.org.uk:4430) you will find a mirror image of your existing site.  This test site is not live, and it is not generally visible.  So you can experiment and try out different concepts without compromising your live site.  The only thing that doesn't work is if you try and send out emails as they do not go anywhere.

Hope that this helps

Alan - Euphonix Choir