What kind of power supply to i need for my gaming pc build? I'm planning to put in GEFORCE GTX 970 and the minimum is 500W they say

Use an online psu sizer app. I use the one on the power supplies page [lower left] at newegg.com.

add 10% for overclocking

Its always good to assume it needs more for any extra devices you might have in the future. A 600 to 650 would be ideal especially if you will be overclocking the cpu and/or the gpu, anything above would make you safe for any future upgrades

For the money your going to put down for the GTX 970… Consider perhaps a GTX 1070

Don't get a GTX 970. They are overpriced and not as power efficient or as good performance as a GTX 1060.
You also need more ram. It should have 8GB, and a hard drive, and operating system.
You need a good efficient power supply more than the wattage.
The stated wattage of a power supply does not mean very much because of the many parameters involved, including whether it is a peak power or continuous power rated.
If you get a GTX 970, you need a GOOD 500+Watt with no less than 30Amp at 12V and 80+ Bronze rated efficiency or better. 80+ Gold is preferred because of the weak air flow of computer brand labeled computer cases with the single cheap system fan. GTX 1060 runs 25Watts lower and 27A@12V is considered minimum with a 430+Watt power supply as workable.
AMD RX 470 is only 12% below a GTX 970 and RX 480 beats a gtx 970
https://www.game-debate.com/...0970%204GB
So, a new GTX 970 is only worth about US$180

The power supply you get depends upon what country you are in.
http://pcpartpicker.com/products/power-supply/#sort=a9&e=6,5,4,2&W=500,2000
For the USA, B+H Photo has the SeaSonic Electronics S12II Series SS-620GB 620W 80 Plus Bronze Power Supply for only $49.90 and at Newegg, a sale ends soon
https://www.newegg.com/...6817153198
on Thermaltake Toughpower TPD-0750M - SLI / CrossFire Ready 80 PLUS Gold Certification and Semi Modular Cables Black Active PFC Power Supply Intel Haswell Ready (PS-TPD-0750MPCGUS-1)
for $50 after rebate.

Because the cheapest decent power supply is $40 after rebate so you spend $50 instead and get a really good one.

In the UK, £51.16 gets the XFX XT600
http://www.realhardtechx.com/index_archivos/Page625.htm
It's made by Seasonic and they are generally excellent.