If you are new to HexaPDF, read this material to get up and running quickly.
If you are new to Ruby, you might wanna have a look at a Ruby tutorial first, to get acquainted with the language. HexaPDF is 100% Ruby and provides a very idiomatic interface. So once you know Ruby, you should feel right at home with HexaPDF!
At the Ruby website you will find all the necessary materials, whether you are new to programming entirely or you already know a few other languages.