First of all, try not to worry about it. I know it's a lot easier said than done since I'm also scared whenever they do something in my mouth what hasn't been done before but getting a filling isn't at all bad. Trust me, I'm not a dentist, but I've been getting fillings since my childhood and I've lost count of how many times a filling has been done or redone.
You can read about the procedure online if that gives you comfort and I can assure you that your dentist does this on a daily basis and absolutely knows what he or she is doing. It's a really simple thing and doesn't hurt at all. If you're like my friend and have very sensitive teeth you can get local anaesthesia even for a filling but normally you don't need it. It twinges a bit but it's over in a few seconds and once the filling is in everything's good. If you're scared of a foreign something being in your mouth, don't be. It'll feel different for your tongue the first day but you'll get used to it so quickly you won't even notice it. I have to look into my mouth to tell which teeth have fillings cause my tongue can't feel it anymore. Your dentist will place it so you won't notice a big difference and if you're still not happy he can grind the filling into a better shape.
If you're worried about having something unnecessary done to your teeth you can always get a second opinion but don't use it as an excuse to delay the procedure. It can get worse, the hole can get bigger and it can start hurting so you'll want to have it taken care of before that happens. Don't worry, getting a filling is an absolutely simple matter. It takes about a minute and it will be over before you know it.
