The most comfortable method to start KDE is to use the startkde script.
Simply put the line
at the end of your
.xsession file (or into your
.xinitrc
file, if you are not using kdm or xdm). Please also remove the lines that
started your previous window manager. If there is no .xsession or
.xinitrc in your home directory, simply create a new one that just
contain one line saying "startkde" (without the quotes!).