The 2x3 connector is an ISP header, it contains reset and the SPI pins. It's used for programming with an ISP programmer (ie, to bootload it). You do not need to use it under normal conditions.
The Nano contains a serial adapter - just plug a USB cable into the USB port. Note that most of the clones now use a CH340G serial chip, not the FT232 that the official boards did - this requires a different driver which isn't included with the IDE (since there has never been an official board that uses the CH340G); google will find it for you.