Start learning the ATMEL AVR. You can find many free online tutorials. But, I think it's better you use a book to learn and use the internet for specifics. This is one of the best books available for ATMEL AVR:
The AVR Microcontroller and Embedded Systems: Using Assembly and C
by Muhammad Ali Mazidi, Janice Mazidi, Sarmad Naimi, Sepehr Naimi