Can we see the code you have so far?
Assuming you're using a RobotDrive object to control your drivetrain, and the tankDrive method, your Xbox controller (like ekapalka said) should behave much like a pair of joysticks in tank drive - with each Xbox stick controlling one side of the drivetrain.
Code:
drive.tankDrive(gamepad.getLeftStickY(), gamepad.getRightStickY();