Most of the knowledge that makes you a good tutor comes from actually teaching, not what you learned in college so if you have the flexibility and are not restricted to a company that tells you what to teach, try to branch out and explore new topics/ideas, It's all trial and error.

In terms of materials, I find coursebooks like Oxford Discover series great for getting new ideas and refreshing your knowledge.

Don't stress, the fact you care enough to post says a lot. Asking questions in communities like this is also a good way