The most important thing of all - tell your dentist, truthfully, warts and all, how scared you are. He or she will be quite used to dealing with nervous patients and may even be a specialist in this area(they do exist).
I never used to like them very much, either, and my first two dentists were awful - no more than butchers, I reckon. Then I found a really good dentist whom I trusted implicitly. I was devastated when he moved away and I went through a string of other dentists, finally finding my current one when I became one of her emergency patients. She's just amazing (and her list is full, unsurprisingly).
I guess what I'm trying to say is, if by any chance you don't get on with your dentist, then it's important to 'shop around' as it were, without feeling guilty for doing so. You need to have a good, easy relationship with your dentist because if you don't like him or her, then you'll be even more tense than ever - and that's gotta hurt!