On systems which give you a special login window or dialog box, the file .xsession in your home directory determines the startup applications. On systems where you personally start X by entering a command such as xinit or xstartup, the .xinitrc file in your home directory is used (as shown by that last ls -a command). In either case the startup files are just scripts, that is, small programs run by the default shell (usually sh). For example, a line in .xinitrc makes the choice between the prettier Motif Window Manager mwm and the simpler but faster twm.
You should check if you already have some of these dot files in your home directory. If you do not, X will use the defaults supplied with your system. For a startup file appropriate to your system, check with your local system administrator or in the system documentation provided by your vendor. You can try the samples given in Appendix 6, although your locally supplied setup file should require fewer modifications in order to work just right.