Although it is possible to develop from the command line tools, if you are looking to develop on or contribute back to any of the sister projects then you should set up a development environment.
Any IDE that supports Maven will do; we use Eclipse along with a number of plugins:
Install Eclipse 3.5 (Java or JEE edition).
the m2eclipse plugin, for Maven support
Normally Eclipse uses its own .project
and .classpath files to (respectively) define
the layout of the project and the classpath for a project. With
Maven however this same information is available in the
pom.xml. What m2eclipse
does is generate the .project and
.classpath files on-the-fly, and uses
Eclipse's own "Classpath Container" to reference Maven modules in
the local repository. m2eclipse will also download any referenced
modules from remote repositories into the local repository.
the Subclipse plugin, for Subversion support
the eclipse-cs plugin, for Checkstyle support
the PMD for eclipse plugin, for PMD support
Install each of these from their respective update sites.