[AVR] Make String Interrupt

#include <avr/io.h>

void serial_init(void)
{
    UBRRH = 0x00;
    UBRRL = 95;   //baudrate 9600 and F_CPU 14745600UL
    UCSRB =  (1 << RXEN) | (1 << TXEN) | (1<<RXCIE); 
    UCSRC = (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)|(1 << UCSZ1);
}

unsigned long long Usart_Receive(void)          
{
    while((UCSRA & (1 << RXC)) == 0) {};
    return UDR;
}
void USART_Transmit(unsigned long c)   
{
    PORTD= 0b00000100;  //RTS Enable
    while ((UCSRA & (1 << UDRE)) ==0) {};
    UDR = c;
    PORTD= 0b00000000;  //RTS Disable
}

int main(void)
{
    unsigned char data;
    serial_init();
    while (1)
    {
        data = Usart_Receive();
        _delay_ms(100);
        USART_Transmit(data);
    }
    return 0;   
}

ISR USART_RC_Vect()  {

}

void serial_init(void)
{
	UBRRH = 0x00;
	UBRRL = 95;
	UCSRB =  (1 << RXEN) | (1 << TXEN) | [B][I](1<<RXCIE)[/I][/B]; 
	UCSRC = (1<<URSEL)|(1<<USBS)|(3<<UCSZ0)|(1 << UCSZ1); 
	
}

ISR (USART_RXC_vect)
{ 
//I dont know what to put

}

ISR(USART_RXC_vect) 
{ 
   char ReceivedByte; 
   ReceivedByte = UDR; // Fetch the received byte value into the variable "ByteReceived" 
   UDR = ReceivedByte; // Echo back the received byte back to the computer 
}