A simple filling honestly doesn't hurt, and it's not that scary!
Some can even be done without an injection of local anesthetic, but that's if it's only a surface filling.
Firstly, they'll put gel; a TOPICAL anesthetic which the dentist will place on the GUM at the injection site (if an injection of LOCAL anesthetic is needed). They'll wait a minute or two, to test that it's working, and then gently insert the needle. You probably wont feel the actual needle, the only part you might feel is the actual anesthetic moving through the gum. It might cause a slight stinging sensation for a few seconds.
The dentist will then put the needle away! and wait.
They usually give you a few moments, and then test to see that the local anesthetic is working, by doing a few different 'tap' tests on the tooth and surrounding gum area.
There's two types of 'numbing' techniques used. One for just the tooth that's being worked on (usually the technique used for simple fillings) and there's a block technique used to numb the whole side of the jaw. (If you're having major work such as an extraction, or root canal, they'll use this technique).
Once they've made sure that the tooth and gum is numb, they'll use a small drill to drill out the 'decayed' area. There's no pain associated with the drilling, as your tooth will be more than numb. You will feel a strong-ish vibration, as the drill moves around and the handle touches your lip or gums. This doesn't hurt, it just feels like you're resting an electric tooth brush in your mouth. Bit odd, but in no way painful.
When they've cleared away the decayed part of the tooth, they'll then start the filling process.
Fillings are actually a paste like substance, which they 'paint' onto the hole, or surface of the tooth. It usually goes on in quite a few layers, depending how deep the cavity is, to be filled.
When they've finished filling it up with the paste like substance, they'll shine a UV light onto the tooth, which 'seals' the paste like substance onto the tooth's surface. They'll then ask you to bite on a small piece of black 'blotting' paper, and presto! You're done!
It's really as simple as that!
Remember, if you've got ANY pain, at all throughout the procedure (even though you shouldn't), make sure you raise your hand, and alert the dentist or assistant.
They can then top you up with more local anesthetic as needed, and once again, wait and test to make sure it's working!
If you're extra nervous (like I was the first time I had dental work done a few months ago), it's a very good idea to get Nitrous Oxide. It's actually what you'd know as Happy Gas! It's brilliant stuff. It doesn't send you to sleep, but it relaxes you to no end. A few deep breaths BEFORE the needle, will do absolute wonders.
I used to be so scared of dentists, and I didn't go to one for 10 years, because of my fear- but I ended up having three periapical (root) abscesses, which all required an extraction of the three teeth.
I was shaking, teary and ready to bolt on that first appointment, but Nitrous Oxide, and a really nice dentist got me settled, and now- i've been to the dentist 14 times in a year now, for different work and now orthodontic work, and I couldn't be further from a fear.
Don't worry, you'll be fine :)