Ruby is great language, especially if you are learning how to program, and comes already installed in Mac OS X.
It can do if-else, for and while loops, booleans, functions, objects, and anything else you would expect. It's known for being very readable and for allowing incredibly short and concise code.
For text editors, there are a lot of options. I like Smultron and Komodo Edit - the first is just a very simple text editor with syntax highlighting, the second is a complete developer environment.
Ruby code doesn't have to be compiled, because it's an interpreted language. You can execute a ruby program by entering "ruby name_of_program.rb" in Terminal.
If you open Terminal, and type irb, you can start writing ruby right away. IRB is an interactive ruby shell, where you can type and execute ruby one line at a time, and see the results as you go along. It's useful for playing with ideas in ruby code and for debugging ruby.
Here is a quick start guide -
http://www.ruby-lang.org/en/documentation/quickstart/
This ruby koans. It's kind of an interactive puzzle you have to solve with ruby code, that teaches you how to write ruby as you do it. It's very interesting approach to learning a language. It took me many hours to complete.
http://rubykoans.com/
There is a vibrant community of ruby developers online eager to help eachother out, and many of websites and books devoted to teaching ruby. You can usually find the solution to any ruby problem you run into by Googling it.
https://www.google.com/search?q=ruby+programming