I am amazed that you want to start with a USB connection when all appearance seems to point to the fact that you are very first beginning your encounter into embedded development. Why not instead build your 8051 with a standard RS232 port. Then you may connect the 8051 to the PC by way of COMM port.

USB is cool, but getting a basic understanding of embedded systems programming is a lot easier starting with the UART on the 8051. After you feel like a pro with that then you will be ready to tackle the USB part of the project.

If you still must have USB at the computer then you should purchase one of these cables which will easily allow you to connect your 8051 RS232 port to a USB port at the computer.