MEGA 2560 Help wanted with coding

Hi evryone,
still not found help

Can you please advise how i might send a byte of data via the usb to the mega
(received from a pc)
which will drive 8 digital outputs.

As I also intend sending another stream of data, is it possible to add an identifier to the data so that the mega knows where to direct the data ie write digital out data to a variable ready to be used.

I plan to use the same port to transmit serial data for dmx output to the mega hence the requirement for an identifier.

Regards
Jason