Hmm...if you want to see data on a circuit, you could use a protocol analyzer. I guess I will wait to recommend a brand/type until we get a little more information. There are a lot of smart people here, and with some more specifics of your needs, I am sure you will get a suitable answer.
An electronic circuit is nothing but interconections of varius components. So to begin with, you should get good knowledge about them, then some good knowledge in electrical and electronics basics.