Connection of serial pin on ATMEGA328P to connector B type(female) D+ and D-
The Arduino Uno (and similar boards) have a chip on them that does serial communication via a dedicated USB chip (the ATmega16U2 chip). You can't just connect pins 2 and 3 to D+ and D- and turn that into USB.

You can conceivably do it using V-USB which does USB in software in conjunction with a few extra components.

You can also buy external serial to USB adapters which do the conversion externally. If you Google serial to usb adapter you will find quite a few for around $3.