The PSU or SMTP is one of the most important parts of a PC. This is also one of the most ignored component by system builders while buying PC parts. It is the Power Supply Unit that takes in high voltage power from your house socket and converts it into low voltage power that is then supplied to various components inside your PC.
Always, always buy a PSU from a well known brand that has good after sales support. Also, you should select and buy the PSU after you've selected and bought all the other PC components.
PSUs come in different wattage, they range from a minimum of 450w to a maximum of 1200w. Depending upon the power requirements of your PC, based on the components you buy, you need select the appropriate wattage for the PSU you buy. Having a 1200w PSU for a PC that needs about 650w is not just waste of money but also waste of power.
Please find a URL mentioned below that will take you to an online calculator. This power wattage calculator will ask you for the PC components that you've selected and will calculate the total power requirement for your PC. You can then go ahead and buy a PSU with the relevant wattage from stores.
I would personally suggest you to buy a Corsair branded PSU. They are a well known brand and have good after sales support. There's also Cooler Master branded PSUs that you can consider. I, myself use a Cooler Master 650w PSU for my desktop and have been very happy with it for the past 3 years.