How do I figure water flow having the diameter of pipe and PSI?

Actually, after having re-read your question, I realize I didn't answer it at all. I mis-judged your apparent level so I deleted my answer.

You are given the pressure and the cross-sectional area. If the flow is laminar, this is most easily solved using Poisuelle's equation (Hagen-Poisuelle Equation). I won't write it. It's probably best just to show you the references. It requires a knowledge of the dynamic viscosity (Newtonian fluids only, I believe), the pressure, cross-sectional area, length of the pipe (and there can't be any bends in it or you need to make corrections). The most important thing we get from Poisuelle's equation is that laminar flow is proportional to the fourth power of the diameter.

http://en.wikipedia.org/wiki/Poiseuille%27s_law

http://www.rwc.uc.edu/koehler/biophys/3c.html

Turbulent flow is more complicated, and I don't, personally, know the equations for that. Chemical engineers have a lot of semi-empirical methods they use.

http://www.pipeflowcalculations.com/