Learn about controllers and actions in Rails. I would recommend the
Agile Web Development with Rails or Ruby for Rails as good books to
Eventually you'll learn about the link_to helper you can use in your
views, for example:
<% link_to 'Complete This To-Do Item', :action => 'completed' %>
and you have to implement the corresponding 'completed' action in your
controller. But that's a lot to explain here, so start with one of
the books above.