Hye sanju kumar,
Before explain anything, i think you know basic concept of C programming. First learn the I/O device interfacing with AVR like led interfacing, keypad interfacing, LCD interfacing etc., After you can go on your application.
Use the modular approch to develop your application.
– Devide your application in subtask like how to interface lcd
– Write code for Interfacing device
– Run and test the code on hardware(i.e. development board or breadboard circuit )
– Solve each bugs
– Now make flowchart and algorithm for your application
– Prepared Circuit diagram
– Write code for your application
– Run and test on your application hardware
– Find the bungs and solve it.
Note : In EngineesGarage , Lots of C programming tutorials and also various small interfacing tutorials available for various controller. You can learn from it. It will be helpful for you.