WPILib is developed by volunteers at Worcester Polytechnic Institute and is a library of classes that help make FRC programming eaiser.
WPILib provides 4 frameworks for building robot programs. In GettingStarted, we built our first programs using the TimedRobot (previously had been IterativeRobot) framework . For competition, 2537 uses the command based robot framework.
You can learn more about "FRC programming done right" here
ScreenStepsLive describes the process; you can also follow some video tutorials: here or here or here or
STEMRobotics has a nice tutorial here
Building your first robot with RobotBuilder or see here
Video tutorial: your first (iterative) robot program here
For the brave: a 1-hour comprehensive Crash Course in FRC programming in Java.
FRC World - the simplest robot example
You can learn more about game controllers