Is your data text or binary? If the former, see Robin's reply.
If it's binary, you need to design a complete protocol that will contain a start marker, a length indication, the data and a crc so you can check that the packet is indeed a packet; a timeout might also be useful.
Regarding the start marker, it depends on the type of data that you want to transfer; if it's e.g. values obtained with analogRead(), 0xFFFF can be used. If it's status of switches, you can transfer 7 switches in a byte and the 8th bit can indicate start of packet; in which case the other seven bits contain either the status (start marker 0) or can contain the length of the message (start marker 1). Your creativity is the limit 
What is at the receiving end of the USB?