Go to the Yahoo! Go 3.0 site. Help for registering a Yahoo! ID is available there if you don't have one, as well as all the prerequisite steps that apply to your device, including examining compatibility and memory requirements.

Notes:

• We send an SMS message that includes a link which you click to open a WAP page in the browser on your device. If your device does not support clicking on links in SMS messages, please open your device’s Web browser and navigate to http://mobile.yahoo.com/go. If you can click the link but your device still can’t download the application, then check the device for correct WAP settings. You can get the correct WAP settings for your device from the manufacturer's web site.
• In some devices, it is necessary to upgrade the firmware before you can install Yahoo! Go 3.0. When your mobile device's browser is preparing to download Yahoo! Go 3.0, the firmware version required is listed in the details about the download. To upgrade your device's firmware to the listed version, refer to the device's manual or contact the carrier or manufacturer.
  In addition, some carriers might not have the firmware version required. In these cases, updating the firmware only results in a message that your device already has the most recent firmware. At this stage, only your carrier can remedy the problem.