Welcome to the foundation for your Echo Nest application.
You will want to create a settings.rb file with the following:
API_KEY='yourechonestapikey'
SECRET='somekindofrailsappsecretthatisalphanumericandverylong'
Get an Echo Nest API key by visiting the (Echo Nest Developer Site)[https://developer.echonest.com/account/register].
You can generate your SECRET on your own. Look in previous Rails apps that you have made for an example.
Try a few methods out once you have your settings.rb file set up...
s = Song.new
s.search(:artist => 'green day')
a = Artist.new
a.search(:name => ['green day'])
Additional methods and documentation with arguments can be found in
app/models/song.rb
app/models/artist.rb
That's it! Have fun!