You need to decide on the Wattage required for your build. Input it in http://pcpartpicker.com/parts/partlist/ and it'll give you a "max load" estimate at the top.
Different configurations have different PSU requirements. AMD parts will generally pull more than Intel/nVidia components, but the differences can be huge, or minimal.
From there, refer to the Tier list
http://www.tomshardware.com/forum/id-2547993/psu-tier-list.html
Consideration then needs to be Modular vs Non-Modular. Modular is easier for cable management, but non-Modular is generally cheaper.
Tier 1 & 2 are quality units & highly recommended - just find the one that meets your Wattage/Price needs and go from there.
Tier 3 are somewhat ok, and work on a budget, but you can generally find Tier 2 units for around the same price.