If you’re using Mac OS X, I think TextMate is (still) a good text editor to start with. It will not grow with you, when you outgrow it I recommend RubyMine (which I love personally)
Other popular choices include Sublime, or many developers like vi.
Here’s a stack overflow post about different text editor options: http://stackoverflow.com/questions/16991/what-ruby-ide-do-you-prefer
I’d recommend you spend some time reading the manual of whichever one you choose, and in particular look for the section “How to Create a New File” which will answer your question in the context of whatever text editor or IDE you are using.