In order to keep your code organized, especially with bigger numbers of team members, you need to have a source control system. There are all sorts of version control systems starting from CVS passing by the famous SVN to the new distributed version control systems such as Git and Mercurial, which offer a different model, particularly for offline usage.
Git is a free and open source distributed version control system; you can learn about it very easily at http://git-scm.com/documentation.
Git also has a tiny footprint with lightning-fast performance. It outclasses SVN, CVS, Perforce, and ClearCase.
You don't have to follow the trend in here, just use what you and your team feel comfortable with.
I chose Git as ...