What is exact your question?
Do you want to know code, how to configure the usart, c-compiler,or how to start a project like this?
You need a C-compiler like Codevision Avr ( https://www.hpinfotech.ro/ )
Configure the usart with the Wizzard (speed, start/stopbits/parity/timers/interupts/IO/ etc) and write the rest of your c-code.