To find your iPhone's UDID, just plug it into your computer and wait until iTunes recognizes it. Select your phone from the Devices list in iTunes and click the "Summary" tab. To see your UDID, click on the word "serial number" beside the picture of the iPhone. You should see the word "identifer" and an alphanumeric string – this is your UDID.
To copy it, just press command + C on your Mac's keyboard (or control + C in Windows). Now comes the hard part: getting on a developer's beta testing list.