Does getting a filling hurt?

I am going to the dentist tomarrow and i am getting a filling on one of my top row teeth. I got a filling a long time ago on a baby tooth but all i can remember is that they used gas instead of giving me a shot. I am terrified of the dentist! and after the filling i am going to wait for like an hour then get my teeth cleaned. Does the shot hurt really bad or is it less painful then getting a shot in your arm? Help!!!