It's not scary at all and you have nothing to worry about while having it done, as long as the person is skilled to do it and they use sterilised equipment! I think they have to hold some kind of certificate, check and see if there is one on the walls?
Anyway, from what i remember having mine done is that they will spray a cold spray that "freezes" your belly button are, a bit like an anaesthetic! This is so you don't feel any 'discomfort', then they use this tool that looks a bit like a salad server!! this is to clamp two folds of skin to make the piercing, lastly they insert the bar through a sterile tiny plastic tube, they screw the ball on and voila it's over! They'll put a covering over it, like a plaster, this is just to protect it, it will be sore when you sit down and wear loose clothing around that area for about a week. The piercing process itself is a few seconds. The main thing you have to worry about is making sure you keep it clean and don't let it get infected (i've heard some horror stories before where the person has not 'cared' properly for it!) If the place is good and the people know what they're doing they will atell you about after care, how to clean, usually you can buy a spray that you can use in the morning ands at night! But you can also clean it with cool boiled water and cotton wool!
Hope this has been helpful to you and good luck! I had to take mine out when i was pregnant and when i tried to put it back in it got infected.