Carefully follow this guide step by step and you should be okay:
https://wiki.archlinux.org/index.php/Beginners%27_Guide
If you still have problems after that, then tell us more details. Since you have provided none, I'll assume you simply haven't installed a GUI yet (since Arch doesn't come with one by default---it's very different than other distros you may have used in the past) and that is the cause of this error.
