how to match /posts/123

I have a postscontroller, with a show method:

def show

end

I want to match the following url pattern to this controller action:

/posts/123

Shoudl go to the postscontroller, show method with id=123

Have a read of this:

http://guides.rubyonrails.org/routing.html

It explains all about the rails routing system.

As I guess that you are a beginner with Rails I suggest that you work
through a good tutorial on Rails so that you will understand the basic
principles. railstutorial.org is good and is free to use online.
Make sure that any tutorial you try is for rails 3 and make sure you
install the exact version of rails that the tutorial is based on.

Colin