You make no mentioned of amperage needed. That being said, a pretty easy way is to pick up a computer PSU (that will provide your 12V and 5V just make sure your needed amperages are met. As for the 7V, you can easily put a 7V regulator and tap that off of the 12V line. There are your power sources. As for turning it on, you just need to connect Pin 13 on the main connector (usually its green) to ground. That is a sense line so its very low voltage basically no current. You can even cut off the main PSU cable except for those two wires and put your power switch on that.