Surprisingly, most Flight Sim users start wanting to learn how to fly airliners with computers and all the stuff. As you get bored you start going backwards, I mean, start hand flying, tuning radios, making VOR to VOR flights and such things that leads you to learn how the complex airliners systems works.
So as patch said, start with the C172 is good for getting an idea on how radio nav works, and learning to keep an eye various things at a time as the landscape moves slower that in a A320 or B737. Once you get used to the plane, give a try to the GPS with the default flight planner just to start, later you'll find it pretty much useless.
So to sum up... do what patch said, then take the leap to the airliners. Of course if you get bored give a shot to the airliners, but you'll find them a bit more difficult (talking about a default one, addons such as PMDG 737NGX or LH MD-80 are much more complex! )