Hi
Have a look at this event www.westwight.co.uk/triathlon It's a great event for first timers because the organisers go to great lengths to ensure that all the marshalls have lists of competitors names and numbers - so you get cheered on by them all!! It's also very friendly, but efficient.
If you've done a tri before then don't miss this one, because it's also challenging enough to be worth the trip to the Isle of Wight.