How do I choose a Power Supply Unit for my PC?

you need to select a power supply with the appropriate connector to your mother board. A 24 pin, a 20 pin or a 20 + 4 pin. you should also use a PSU calculator to ensure you are getting the appropriate size PSU. Google PSU calculator. Also you need to be aware that some PSUs are meant to be mounted in the bottom of the case while others are meant to be mounted in the top.