There is a lot of good material on instructables.com. Try searching google for garduino, gardening + Arduino.
Arduino code is pretty basic. There are some great tutorials on the web but it can be difficult to figure out were to start. I found the Vilros Starter Kit to provide a good introduction. It is $55 on Amazon and comes with some components. I really like the tray that a bread board and Arduino can attach to, it makes developing so much easier.
http://www.amazon.com/Arduino-Ultimate-Starter-Includes-Instruction/dp/B00BT0NDB8
Most solenoid valves need more pressure than a tank of water can provide to work. There are 0 pressure solenoids though. This link has a discussion about it.
http://forum.arduino.cc/index.php?topic=238723.0
For my project I ended up going for a small half amp pump since my goal was to have everything run off of a solar panel.
Also if you are working with water there is a quick and dirty moisture sensor that you can use to detect a leak and shut things down. Among other things this link has instruction for making the sensor and the code.
http://makezine.com/projects/make-18/garduino-geek-gardening/
Happy tinkering!