You first set-up your google apps account here:
http://www.google.com/a/cpanel/domain/new
Then you have to modify your MX records for the domain:
http://www.google.com/support/a/bin/...n&answer=33352
Then, if you want to access your email in a url like this: mail.yourdomain.com, you need to set up a CNAME record:
http://www.google.com/support/a/bin/...n&answer=53340
I am not sure how your DNS control panel from one.com looks like, but it should allow the above modifications.