The DC-9/MD80/717 are all fundamentally the same aircraft/airframe and are very easy to recognize with their two small tail mounted turbine engines.

The 727 is actually very different; it has three tail mounted engines. You probably don't see these very often.

You should play on airliners.net and check out the photos for a while.

It's pretty easy to distinguish between a 737, 757, 767, 777 etc and then you can start working on the A319, A320, etc.

Then take on the smaller, private planes. Have fun.