how to install Ruby on rails webshop?

You have installed RoR3 and now you want to install a rails3 webshop.

I think the best solution is Spree. This is the only rails3 based e-commerce solution at this time.

How do you install this shop?

rails new rails3-shop
cd rails3-shop

edit Gemfile and add new line

gem 'spree'

Install new spree site.

rails g spree:site
rake spree:install
rake db:migrate
rake db:seed
rails server

The rails server command starts the webrick server. You should go to http://localhost:3000/ to check spree.
The first registration will be the admin user.