how to read data on wire

If you are only lookging for 12vdc wires you can probe with a meter.  If there is data on the wire, it will NOT read 12vdc.  Data wires will usually read under 3vdc with a normal meter.

A 'scope isn't exactly the easiest piece of equipment to use.  You need to make sure a lot of things are set correctly to measure what you are trying to measure, starting with time period and voltage level.  There will probably be channel selections and trigger modes that if not set right won't allow you to use the 'scope.  Your best bet would be to read the book front to back and understand it all.  I've had my 'scope for 3+ years, and honestly, I don't know how to use all the features on it.

It would actually depend on the signal, but I would say that would be a good generalization.

Most data signals are 0-5vdc signals, however you will occasionally come across a 12vdc 'data' signal, like a fuel injector signal (technically is a data wire) or even possibly a crank angle signal). 

I would say, more importantly, just don't tap in to any wire that doesn't read a constant 12vdc when you probe it.  Also, use a good wiring reference for the wires you need to tap.  You should never go in to a car without and idea of the color and location of the wire you intend to tap in to.  Once you have that information then you verify it with your DMM.