There are many programs that will walk you through how to teach. For example, Math-U-See will teach high school math for you, and All About Spelling is scripted so you know exactly what to say. Other programs will provide extra notes for the teachers or online support or help lines.
You can learn alongside your child, hire a tutor, or use a program that will teach for you. You aren’t required to be an expert in every subject. Or even any subject.