The majority of the book is Rico going deciding to be a trooper, why, getting schooled on why the government system works the way it does, and the experience of him going from naive rich kid slacker to hardened soldier, hero, and leader. Something like 80 to 95% of the book is essays on Government, Rights, Ethics, etc.
So just about everything is changed from the book to the movie considering the movie focuses on boot camp and then 1 or 2 mission, one of which everything goes wrong with little to none of the understanding of the why or how of anything. In other words, all the important intellectual stuff was removed to provide a action comedy (brainless humor) more or less. I would say that the movie is more of a Parody of what someone who doesn't understand what the book is about made.
All that being said... Go read the book. It's one of my 5 must reads and should be forced on everyone to read at some point, and enjoy the movie for what it is. You might also want to read everything else by Heinlein too, because while they aren't the same universe, it will help understand Starship Troopers and put its world in better context.