FVWM and FVWM95 are two public domain window managers which are very stable,
fast, featureful and at the same time have a relatively small memory footprint.
Which one you choose is a matter of personal taste, since they both have the same
core functionality and features (and share more than 95% of the code). FVWM95 has
a few more bells and whistles, as it tries to provide the look & feel of a certain
popular PC windowing environment.
If you will be working on an Xterminal or a workstation you should
look at the man pages for xinit and X. The best way to
set up your environment is to speak to other users and find out what
they prefer, then read other users' configuration files for tips.