Have you ever owned a computer before?
Have you ever plugged a mouse or keyboard into the back of the computer before?
That piece on the back of the computer with all the various ports/connectors that looks like this:
That IS the motherboard's rear panel...it's the default point of connection for keyboard/mouse/USB. those are built into the motherboard. In order to use the ports on a case, you have to physically connect wires to the motherboard during assembly.
Have you ever built a computer before? While it's not terribly difficult...it is detail-oriented work...and based on this last question, MAY be above your skill set.