Before diving into the code, you should first take the time to make sure you have an environment where you can run puppet as a developer. In a nutshell you need: the puppet codebase, ruby versions, and dependencies. Once you've got all of that in place you can make sure that you have a working development system by running the puppet spec tests.
In order to contribute to puppet you'll need to have a github account. Once you have your account, fork the puppetlabs/puppet repo, and clone it onto your local machine. The github docs have a good explanation of how to do all of this.
Puppet needs to work across a variety of ruby versions. At a minimum you need to try any changes you make on both ruby 1.8.7 and ruby 1.9.3. Ruby 2.0.0 and 2.1.0 are also supported, but they have small enough differences from 1.9.3 that they are not as important to always check while developing.
Popular ways of making sure you have access to the various versions of ruby are to use either rbenv or rvm. You can read up on the linked sites for how to get them installed on your system.
Make sure you have bundler installed. This should be as simple as:
$ gem install bundler
Now you can get all of the dependencies using:
$ bundle install --path .bundle/gems/
Once this is done, you can interact with puppet through bundler using bundle exec <command> which will ensure that <command> is executed in the context
of puppet's dependencies.
For example to run the specs:
$ bundle exec rake spec
To run puppet itself (for a resource lookup say):
$ bundle exec puppet resource host localhost
Puppet Labs projects use a common convention of using Rake to run unit tests. The tests can be run with the following rake task:
bundle exec rake spec
To run a single file's worth of tests (much faster!), give the filename:
bundle exec rake spec TEST=spec/unit/ssl/host_spec.rb