The order of forking and branching doesn't matter, but it is typically easier to:

  *  Fork
  *  Clone your fork on your local machine
  * create a branch `git checkout -b SEC-2002`
  * do your work
  * commit your code `git add .; git commit -m 'SEC-2002: did some things'`
  * push your code to your repo `git push origin SEC-2002`
  * create a pull request from your branch

You might read the contributor guidelines which points to github doc. Alternatively, this is a rather streamlined article that might help get you going http://blog.springsource.org/2010/12/21/social-coding-in-spring-projects/