Changing window managers will make major changes to the look and feel of all your windows. It is not advisable to do so unless you are sure you understand the consequences.
You do not need to do anything if you want to use the default Athena window manager (metacity for debathena). Due to significant changes in the debathena environment, we do not recommend that you use window managers in th windowmanagers locker or others installed elsewhere, but rather use the optional ones accessible via the Session: button on the login screen only.
Changing windows managers on a machine supporting a vendor-enhanced manager may cause you to lose all of the vendor-specific features like floating toolbars and icons. In some cases applications will crash if you use a window manager different from the one intended by the software vendor. Many window managers allow you to customize settings with a configuration dot file that should be put in your home directory. If the window manager is called foo, this configuration file is typically called .foorc and templates called system.foorc which you can copy, rename and customize are usually located somewhere in the window manger distribution tree (see documentation for details). You might also want to look in the dotfiles locker for ideas on how to customize settings.
In a number of cases, the window managers support correct basic functionality but utilities and program launchers may not be configured or present on Athena; consequently attempts to launch these using window managers controls may fail. Some window managers may start processes that can add significantly to your cpu load. These may need to be killed manually if you decide to switch to another window manager. Window managers are normally started automatically on login provided certain entries are in required configuration files. You should not need to start one manually.
Additional information is at the Window Managers for X page.