How can I easily connect to my breadboard circuit using USB?
If you have a USB transceiver already on the breadboard and you just need to connect USB somehow, they make little breadboard adaptors like the one pictured below:



You can find these things buy Googling "USB breadboard adapter."  An example is here.

If on the other hand you do not have a USB transceiver and would like to communicate with your circuit by a UART port, then you need a similar item, but with a USB to UART converter chip:



To find items like this, Google for USB FTDI Breakout board.  An example is shown here.  After installing the FTDI Virtual COM Port driver this type or board will show up as a COM port on your computer.