What knowledge and skills should teachers have to ensure effective teaching?

You should first be sure to have good communication skills to be able to clearly tell students what you want them to do. Depending on what grade you're teaching or what specific subject you're teaching, you'll need different knowledge, although if you are are teaching in lower grades, a brief understanding of each subject could be helpful since younger students may go to other teachers for help with another subject. But mainly good communication skills and also the ability to pick things up quickly since sometimes you may be learning something completely new just lik the students and you'll need to tutor yourself on it if it's new to you.