I've covered this at more than sufficient length elsewhere, but I still get asked about lighting in Iray, so I wanted to do a quick set of text basics for those who have trouble ferreting out the relevant bits from the videos.
Here is a reference set of lights and a Sunny 16 preset you can download and use for practice. These are the lights I use as starters in 90% of scenes now. If you want to know more about tone mapping I've gone into that here as well.
THREE-POINT LIGHTING: WHAT IS IT?
Wikipedia covers the photographic theory of three-point lighting here, if you would like to know more about that. The short version is