Most of the times, when working with Github, you will want to see the differences between the current local copy of a file and the previous commits. One of the easiest ways to see it visually would be to make use of the gitk command and give the file name as the argument. e.g.
Thats all. This command would open up a neat little window and show you all the changes that were made to this file across different commit and lets you compare it with the version in your current working directory. Oh, and do remember to include the relative path before the filename. Standard convention.