The Pig Documentation provides the information you need to get started using Pig.
Begin with the Getting Started guide which shows you how to set up Pig and how to form simple Pig Latin statements. When you are ready to start writing your own scripts, review the Pig Latin Basics manual to become familiar with the Pig Latin operators and the supported data types.
Functions can be a part of almost every operator in Pig. The Built In Functions guide describes Pig's built in functions. The User Defined Functions manual shows you how to how to write your own functions and how to access/contribute functions using the Piggy Bank repository.
The mechanisms featured in the Control Structures guide give you greater control over how your Pig scripts are structured and executed. The Performance and Efficiency guide provides valuable examples and suggestions for optimizing your code.
If you have more questions, you can ask on the Pig Mailing Lists.