When should I quit my job?

I have worked at my current employer for 2 years. The past 2 months I have been taking days off to substitute teach. I am going to take the Praxis test next month to get my teaching certificate. I am scheduled to substitute this week but I don’t have the guts to tell my boss that I will be out again without her thinking that I have another job. Eventually, I do want to leave. I think what is holding me back (other than the fact that I don’t like to let people down) is that I have a family to support. On the other hand, I can not stand my job and I want to start to follow my dreams. When should I quit my job and how?