Check out all the examples - it's a good place for idea's and plenty have a login screen. Bruce also sells a book "Building Web Applications using Nettalk" that you can get from clarionshop - it is worth every penny and wished it existed when I started out on Nettalk. Would have save me many wasted hours...
http://www.clarionlive.com/ also has a few free Nettalk webcasts you can download for free which are very informative. I think Clarionshop also has some training videos for sale from some of the devcons.
Always good to start small on your first project. If you get stuck, search the NG as the Q has probably been asked/answered before and if not post your Q
HTH's
Kevin