Take a look at the following link, it should help and is the source of the diagram below:
http://www.rowand.net/Shop/Tech/AllAboutRelays.htm
All automotive relays are wired similarly, and everyone that I have seen use this numbering code for the wiring. Think of a relay as an electrically controlled switch (which is exactly what it is), that allows a small current flow to control the flow of a high current through it.
As the image shows, the main power from your battery (fused close as possible to the battery) will connect to #30 on the relay. Wire the line for the trigger (#86) to a source that is on when the car is on, such as your radio, lighter, axillary fuse, amp turn on line, power antenna line, etc. You can safely splice into this line, as it will not be carrying much current. Wire the ground to #85 (it does not have to ground to the same location as your audio equipment and carputer, but it can). The wire carrying power to your fusebox will connect to #87, the "normally open" connection.
When the relay is energized (when the ignition is turned on) with the current from whichever circuit you choose, it will close the relay, allowing a higher voltage current to flow to the fusebox.
This type of power controller works very well with a laptop, as you can turn the car off and the laptop can continue its shutdown procedure on its own battery power. No waiting for the computer to finish shutting down or hibernating before turning the car off and getting out.
Let me know if you need more help