Prerequisites

• install Git on your computer (Tutorial@git-scm.com, Tutorial@github)
• configure git for github, see
  https://help.github.com/articles/set-up-git
  git config --global user.name "Your Name Here"
git config --global user.email "Your mail here"
• install Java SDK on your computer (Java 1.6 tested)
  
• install Eclipse JEE (tested Versions: "Indigo 3.7", "Juno 4.2") 
• alternatively use IntelliJ IDEA, Oregami has on open source license key for it
  

Source code

• at Github make a fork of the oregami repository
• clone your fork git repository with this command line locally and run the Spring Boot main class org.oregami.OregamiApplication
• open your browser and go to http://localhost:8080/

Development tools

I also suggest you use some kind of "Git client" to see which files are changed/staged and to compare files.
Here are some tool suggestions:

• GitX (OS X, GPL)
• SmartGit (Windows/OSX/Linux, Free for non-commercial use)
• SourceTree (Windows/OS X)

There is also a Git plugin for Eclipse called EGit, but I prefer using Git on the command line and GitX or something similar outside of Eclipse.