Tooth hurts really bad, help! :(?

For a while now one of my teeth towards the back has been hurting. It's gotten to the point where sometimes when i'm eating it'll hurt with every bite i take. I know i have to go to the dentist but im afraid of what it'll turn out to be. What if i have a cavity or need a root canal?? :( if i do, does anybody know how bad it hurts to have those fixed & what needs to be done?