General principle: with Subversion, you have access to a repository inside which you can insert your project files. Subversion keeps track of all the changes you make to your files; all the different versions of the files you submit correspond to revisions. You usually access the most recent revision, use it, make change to it and then submit a new version. You can also obtain files in the state they were at any past revisions. The Subversion repository can be located on your local machine or on a network server where multiple can access it.
Her is a very short introduction to Subversion, the tool you need for your versions control. Simple to use, open-source and multi-platform, check the Subversion website for complete information and installation instructions. You should also install TortoiseSVN which is an excellent Subversion client; it greatly facilitate the use of Subversion in your projects.