Do I have to? Yes and no. Part of your training is learning how to pass along the information once you get to a certain level. This does not mean that you are required to run classes of your own if you don't feel a calling to though. So yes, you will have to learn to teach karate to some extent. But no, you do not have to run your own classes if you don't want to.
When Should I start? It's a very gradual process. First you pair up with less experienced students so they can benefit from your knowledge. Then around mid-orange belt your Sensei may occasionally ask you to review a simple concept with one beginning student. Around high green-blue belt your Sensei will ask you to teach a small group. It isn't until purple and brown belt that you really start helping out regularly.
What's in it for me? Aside from Kotaku, the martial artists' responsibility to help others whenever possible, you will get a lot out of teaching karate. You will gain perspective and valuable practice of your basics and beginning kata. You will learn from your students' successes, their mistakes, and their questions.
Advice? Remember that a Sensei's role is as a guide, the students should do the work and you help them along. Always teach by showing what you want the students to do. Karate is a very visual art. Be confident!