I was posting about this just months ago lol. I was trying to avoid doing a root canal for almost a year. When I finally made an appointment, I panicked and changed my mind at the appointment. Bad idea.They insisted I go before the cavity reaches my nerves because then it will start to hurt. Eventually it did, and the pain was awful, which is when I knew I had no choice but to do it. If you go to a good dentist (check their reviews online) it should be a painless experience, aside from, as Haku mentioned, the initial pinch from the syringe they use to numb the area. The area around your tooth will probably feel sore too. If the dentist is telling you with a certainty that you need a root canal, then yes you'll need one. You don't want to wait til a cavity gets worse.
On a tangent note: The part that I was confused about was the crown, but I guess it's because I didn't do my research on it, and some doctors don't bother to explain exactly what they will do until the day of. After my root canal, I had a follow up appointment for a crown (molding the tooth), and another appointment to place the crown, and at that appointment I asked my dentist what she's planning to do. She says I'm going to shave off the majority of your tooth and place the crown (the molded shape of my original tooth) onto the filling. I was mortified by what she just said lol. I said 'but I feel like my tooth is sturdy.' She said it's hollow on the inside. I asked if people have done root canals without getting a crown, she said yes, that's a possibility (yeah, she tells me AFTER I ask!) but there are risks, such as chipping of the tooth, etc. I said well I'd like to keep my tooth today, and see what happens from there, thanks! I said I'll just avoid biting with that particular tooth lol. As of today (it's been a few months since the root canal) my tooth does feel sore, but it's ok so far.
So, which tooth has the cavity? Mine was the first premolar.