If you have Android: if you have Android 4.0 (Ice Cream Sandwich), or don't have WiFi available, use this manual guide: http://j.mp/giffgaffandroid Otherwise, on WiFi download the app "giffgaffAPN" from Google Play/the Android Market. Open it and install the giffgaff settings (use Option 2, then Option 1), then reboot.
If you have BlackBerry, your internet should work automatically. If it doesn't, try pulling the battery out of the phone while it's switched on then reboot. If you would like to use internet on your BlackBerry without the BlackBerry addon, see here http://j.mp/bboperamini