First, you need to learn PigLatin language. The best source for learning PigLatin is:
Be sure to sign up pig mailing list.
Then checkout Pig source code using svn:
svn co http://svn.apache.org/repos/asf/pig/trunk
Set up environment for Eclipse.
Learn more about Pig internal at Pig paper at VLDB 2009.
Learn the process to submit a patch in HowToContribute.
Browse through Pig code. Some good start points are:
Wanna find something to get start? Here is a list of Jiras to warm up.