I’m not a dentist or hygienist, but I have had about 30 cavities filled (I’m on round two for most of them, now). Let me reassure you that loads and loads of people are apprehensive of dental work, including me. Dentists work fast - you’re out of there before you know it! The anticipation is usually the worst part.
Here’s what to expect - you’ll go in, they’ll take a peek in your mouth, then they’ll take x-rays of all your teeth, to catch any hidden cavities. The x-rays might be painful because of the film that you have to bite down on in your mouth (not too bad, but no picnic, either). Then the dentist will look at the x-rays and go over your teeth with a sharp metal object. She will poke into your teeth and gums with this sharp metal object. Sometimes it will hurt, and that is a cavity. She will also probably look at your gums and see if you have any gum disease yet (at 25, you probably will - take any advice they give you on how to combat it while you’re still young).
The dentist might make appointments with you to have your cavity(ies) filled at a later date, or they might fill it right away, if there is just one small one. When you have the fillings done, the dentist will numb the area of your gum first with a topical anesthetic (which smells terrible), then she will inject the area with an extremely fine needle and novocaine. This is usually the part that gets people. The best thing you can do while you’re getting frozen is keep breathing steadily - don’t start hyperventilating and freaking yourself out. The needles take maybe a minute at the longest - that’s not so bad, is it?
After you’ve had your needles you should be frozen solidly in about ten minutes. When the dentist starts drilling, you shouldn’t be able to feel anything. If you still feel any pain, tell the dentist and she’ll freeze you some more (another needle in the roof of your mouth, usually. This is the worst one, that pinches pretty badly, but again, over quite quickly). Once you’re frozen solid, the dentist will drill out the cavity with a very high-speed, high-pitched, whiny sounding drill. This usually takes maybe five minutes. Then she will cram a bunch of filling stuff in the drilled out area (it might be the white resin stuff, or it still might be silver-amalgam, depending on the dentist). She will then shape the filling if using a silver amalgam, or cure the filling if using the white resin. This takes a couple of minutes, too. The dentist will usually polish the filling up a little also, with what looks suspiciously like a Dremel tool. 
The dentist will then administer some bite tests, to make sure your bite is still okay. These consist of just having you bite down on some inky paper and checking where your teeth are hitting each other. You might need to get your bite adjusted slightly once the freezing’s out - it’s hard to tell how you’re biting when you’re still frozen. Once your bite is good, you’re outta there, baby! I’d say getting a couple of fillings on one side of your mouth takes about half an hour, and I’m hard to freeze, so it takes me even longer than it might take you. Don’t worry! It’ll be fine!
But go have it dealt with. One small cavity now is a helluva lot better than twenty big ones later.
