You might wanna look into the uses of the files ~/.xinitrc and ~/.Xsession to speed you on your way.
For my Gentoo system I have many window managers installed. I boot to runlevel 3 so I my .xinitrc file (in my home dir) gives commands for X to run. The last command is the window manager to load. I have all of the WM's commented out except (of course) for the one I want to use. Here's the current content of my .xinitrc file:
#xgamma -gamma 1.2
xrdb -load .Xresources
#nvclock -m 479 -n 300
Of course, distros behave differently so this is just a rough example.