I'd read the guides and then try to understand how a simple example application works. Then, try to deploy it to Heroku, make some modifications, and see your changes in the real world.
I think tptacek makes a good point, too, though. Learning how Sinatra works and checking out some example apps there might be a good way to start as well.
Here's some links:
http://guides.rubyonrails.org/index.html http://github.com/trevorturk/static/ http://heroku.com
I think tptacek makes a good point, too, though. Learning how Sinatra works and checking out some example apps there might be a good way to start as well.