Oh man where to start with that to help you out?
There are so many distinguishing features between different airliners that I don't think I want to start listing them all here.
Just based on your example of a 777 versus a 737. So much more than the 777 being longer. It's wider, sits higher off the ground, has engines that are pretty much the size of the 737's fuselage, has a flat tail cone versus a rounded one, triple bogey main gears.
With practice you can also start to tell the differences between different versions of the same aircraft. Such as telling the difference between a 737-700 and 800 series (the 800 is longer). Or a 777-200 versus a 777-200LR (raked wingtips).
This page lets you view different aircraft types based on where the engines are on the plane, might be a good place to start and gather info:
http://flightlineaviationmedia.com/planespotting/
When I was a kid I had a copy of Jane's Civilian Aircraft that had all the airliners in it. I would obsess over it and just soak up all the info and pictures for the different types. I'm sure there are books like that still around too.