I have setup google apps for your domain.
I want to provide a friendly way for users to go to the google login page.
What is the DNS record I need to create so that: webmail.mydomain.com forwards to https://mail.google.com/a/mydomain.com ?
In the manage domain settings | service settings | email change the first option "web address" to point to the URL you want (i.e. mail.mydomain.com) then create a CNAME record that points mail.mydomain.com to ghs.google.com.
Assuming you have DNS control for your domain, create a CNAME for mail.yourdomain.com that points to ghs.google.com.
Then go to the Google Apps admin page for your domain and under Email (https://www.google.com/a/cpanel/yourdomain.com/CustomUrl?s=mail) change the URL to the domain name you specified as the CNAME, i.e. http://mail.yourdomain.com.