If you haven't already done so, you'll need to get a working
xorg.conf file. You can do that by going to the command line, gaining root permissions, and running:
Answer all the questions about your hardware correctly, and try
startx again. If it doesn't work, you might need to create an
.xinitrc file. Or if you want to boot right into Gnome and you installed GDM, enter
gdm into the last part of your DAEMONS section in your
/etc/rc.conf file.
This wiki page might help, too:
ArchWiki :: Beginners Guide - ArchWiki