Firstly, in the book a store keeper tried to rob Gordie. He did this by trying to short change Gordie and then putting his thumb on the weight scale. He probably thought that because Gordie was a kid he would not notice. This did not happen in the movie.
Secondly, in the movie Gordie got upset at seeing the body of Ray (the dead kid) but was relativly calm in the movie. This is shown by him basically just standing around. In the Movie it was Chirs the went crazy instead of Gordie. This is meant to make Gordie appear to be a stronger character.
Next, and most importantly, in the book a rival group and tries to take the body,and in the book Chris fires a handgun to scare them off, while in the movie Gordie fires it. This is to make Gordie once again seem stronger. It does this by showing Gordie standing up for the group while Teddy and Vern ran. Because Gordie was willing to fight while others ran he looks stronger.
The many differences like changed or omitted scenes change how you think about the characters. There are probalby dozens of differences that do not really matter such as line differences. If you have not read the the book the differences do not really matter because you do not have opinions of the characters already formed. The differences only matter if you have read the