Goodoil, do you have experience with Ladder Logic and that's why you are talking about PLC's here?
If not, you can figure this out with coding with Arduino.. I strongly suggest "Stepwise Refinement", where you write short test sketches (programs) to run each different valve or function on an off. You can even start with the BLINK example. Then start combining functions together.
There is some information about power switching with Arduino, including relays etc. here: http://arduino-info.wikispaces.com/ArduinoPower
If you are using electrically controlled valves you can operate them with relays or "Solid State Relays". Either way, having "Optical Isolation" between Arduino and the actual power switching is a good idea.
PLC's and Opto22 computer control etc. all normally DO have optically isolated inputs and outputs.
Let us know how it's going...