How much does getting a filling hurt?

I went to the dentist today and was told I have to get a filling. I'm extremely diligent about brushing my teeth twice a day with my prescription toothpaste, flossing every night, all that jazz. And I drink a lot of water and tend to stay away from sugary foods. But anyway, even with all of this, my teeth are just really weak and I managed to get a cavity. I had bad teeth when I was little too, so I've gotten fillings before but I can't remember what it felt like at all. I'm 15 now, and I have an extremely low pain tolerance so I'm honestly scared to death. Does the shot hurt, and if so how much? Also, does the actually filling process hurt? How much? Is there anything I can do beforehand to make it all hurt less? Thanks you guys. :)