Let's do a hello world.
In your Unix/Linux or Mac OS X console,
type in
shell>irb
irb(main):001:0>
Let's do a Hello World
irb(main):001:0> puts "Hello World"
Hello World
=> nil
shell>irb
irb(main):001:0>
irb(main):001:0> puts "Hello World"
Hello World
=> nil