how to utilise diff_drive_controller package in my robot?

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In order to be able to use the diff_drive_controller from ros_control, you'll have to write a hardware_interface that allows diff_drive_controller to communicate with your sensors and actuators. A hardware_interface is basically a C++ class that has read() and write() methods and is a specialisation of a generic hardware_interface and knows how to read sensors and write to actuators (or their intermediate registers / whatever) for your specific hardware platform.

It's an abstraction that allows the generic diff_drive_controller to use your specific hardware, without knowing anything about your hardware.

For a mobile base using a differential drive, the sensors would be the encoders and the actuators would (most likely) be velocity controlled motors.

There are generally two 'types' of hardware_interface implementations:

1. those that use ROS msgs to communicate with nodes that implement the actual interface to the hardware
2. those that do not use ROS msgs, but some other means (direct register access, a UDP/TCP connection, etc)

Personally I prefer type 2, but if you already have nodes that expose topics which carry and accept the data you need (such as your lwheel and rwheel and "2 topics the velocities for the wheels") then a type 1 is obviously a possibility.

See #q263501 for a recent related question.