Wattage - You need to calculate how much wattage your system will need before you purchase a PSU, there is a lot of useful tools online to help you with this, or just ask us if you are unsure.
Efficiency- The efficiency rating is simply the power outputted to your components divided by the wattage drawn from the wall socket. 80 Plus is a certification designed to denote that a power supply is at least 80% efficient at 20%, 50%, and 100% loads. This means a 500W 80 Plus rated PSU would draw a maximum of 625W at 100% load. Please refer to the chart below:
*a more efficient PSU can save you some money on electricity in the long run.
Modularity - Non-modular power supplies have all cables permanently attached VS Fully modular power supplies that can detach cables.
Looks - Okay! The last step for enthusiasts is to look at the visual design of the PSU to find one that matches the color theme of the build. Everyone has their own style and the most important thing how the system will look to you. Sometimes, a PSU can be used as an accent to your rigs instead of a junky black box at the bottom of the case.