One of the most important tools for software engineering is version control (1, 2, 3). Basically it's a method for keeping track of the changes you make to your work. Apple has recently introduced a similar function in OSX Lion, where the operating system automatically saves each version of a document you are working on.
Version control is a magnificent tool for software engineering because it acts like a backup of your work, change something and you can easily revert back to a previous version; and it lets you easily see the changes you are making as a difference between saved and working versions.
Anyway, the creator of Linux also made a revision control system called "Git" (after himself apparently). This is all the rage now, so it's a useful opportunity to learn it. At work I use an SVN based system.
Some instructions: http://gitref.org/