I am petrified of the dentist. How could I make going there easier?

I have been having tooth problems since I was little. I have a cavity now and my tooth is beginning to hurt and the lower jaw is beginning to get swollen. I would go to make things easier, but I am just so scared! I know I'm 15 and too old to be scared of this, but I just can't convince myself to stop. I literally start crying and shaking even before I go into the dentist's office. What can I do to make this experience less frightening?