Firstly you need either a chip that has USB built in like an FTDI USB to RS232 or USB to UART bridge type of thing ( probably the easiest method ).... Or you need a micro controller with USB built in.. But that would require learning about the USB specification and the micro controllers USB peripherals.. USB.org - Documents has the details about the USB standards...
Personally, I think you should go find a USB to UART chip, designing the USB code or even using some premade code will be time consuming, unless you want to learn USB.