Hello,
The very 1st thing you need to do is to label all the other components you have in your rig (Motherboard model, CPU model and socket type, Graphic card model, number of hard drives, quantity of RAM sticks, number of fans installed in your rig, etc.)
In order to make a good PSU choice, select one in the tier 1 or 2 of this list :
http://www.tomshardware.com/forum/id-1804779/power-supply-unit-tier-list.html
Now for the amount of watt needed, you will need to use a PSU wattage calculator such as this one : http://www.coolermaster.outervision.com/
Then, to make sure you have made a good choice, you will need to read and understand this ...