Go to this website and fill in your PC components.
It also lets you plan overclocks, if you are into that.
It will give an estimate of how many watts you need.
http://outervision.com/power-supply-calculator
Write that number down and check for a tier 1 or 2 psu from tomshardware PSU tier list
http://www.tomshardware.co.uk/forum/id-2547993/psu-tier-list.html
Dont cheap out on your PSU, its an important part. You dont want one that dies and takes the graphic card with it, or burns down your house. Just get a decent one
If you have any issues with the power supply calculator, theres a guide on youtube:
https://www.youtube.com/watch?v=D0FBkt789rY
If you are not going to overclock, pick a tier 2 PSU with atleast 500W and you will be safe. You could even do a mild overclock on that.
For a more decent overclock (and futureproof) you could go for a 550 - 650 PSU.
happy PSU hunting!