Cables into your desk from say an mic are normal instrument cables, as are the cables going from your desk to the power amp.

The cables going from your power amp to the speakers are speaker cables and are NOT the same as instrument cables as they are of a different impedance.

The ends of the cable will depend on the input and out put sockets on the bits of kit. Where possible use "balanced" XLR cables as they are hum rejecting.

A multi core is also a good idea. Remember to keep your audio cables uncoiled when in use and away from power cables as this can also cause hum.