Once you have signed up your domain for Google Apps, you need to change the MX records for your domain for enabling you to use it in Gmail.
This can be done very easily using cPanel control panel provided to you with your cPanel Hosting account, you need to login to your cPanel account and use MX Entry option. There you can find the domain you need to point to Google Apps Mail and delete all of its MX Records.
Then you need to add the following MX records with the corresponding priority for your domain:
Priority: 1 : ASPMX.L.GOOGLE.COM
Priority: 2 : ALT1.ASPMX.L.GOOGLE.COM
Priority: 3 : ALT2.ASPMX.L.GOOGLE.COM
Priority: 4 : ASPMX2.GOOGLEMAIL.COM
Priority: 5 : ASPMX3.GOOGLEMAIL.COM
Priority: 6 : ASPMX4.GOOGLEMAIL.COM
Priority: 7 : ASPMX5.GOOGLEMAIL.COM
The above are in full compliance with the official Google Apps instructions and can be used to work with cPanel. For the changes to propagate and take effect, it will require approximately 24 hours. After which your mail will be exclusively handled by Google Apps Mail.
You can contact your Web Hosting Service Provider for more information or if you are having any issues in the same regards.