If you're new to rails I recommend you read a book before you dive into the API documentation.
Here are two recommendations:
After you've worked your way through those books, you'll have an idea of how Rails is designed and where to look for stuff.
Both books are great reference books. If you want to access the Rails and Ruby API's online, make sure to checkout RailsApi.com
If you don't know ruby then these 3 books will bring you up to speed relatively quickly. The nice thing is that you can use "Design Patterns in Ruby" as your primary text and the other 2 as reference texts (although you should definitely read "The Ruby Programming Language" all the way through at some point):
My favourites are: