Debian GNU/Linux uses /var/lib/cvs/ as the default system wide CVS repository. You only need to know the location of the repository the first time you checkout a project from it. You can set your environment variable CVSROOT to point to this when you check out new projects. Alternatively use the -d option to the cvs command.
Once a CVS project has been checked out there will be a subdirectory called CVS in each newly created directory that contains the relevant information about the repository (so you won't need the CVSROOT environment variable or the -d option for any other cvs commands within this project).