AtMega8 as USB to serial converter

hi, I've been looking for source codes and firmware for atmega8 so I could use the chip as a USB to serial converter. please help

See https://www.obdev.at/products/vusb/index.html

Note that implementing USB on a microcontroller without hardware USB support is becoming increasingly difficult (doesn't seem to be compatible with some USB3 controllers, for instance), and increasingly unnecessary (since low-cost microcontrollers with hardware USB support have become more readily available and cheaper.)