How to prepare enough material for my class?
It is a common misconception among beginning teachers (including me when I first started) and some experienced teachers that it is good to include more materials in your teaching.  However, since students have a limited ability to learn, it is better to keep the quantity of material within that limit.  Pick the most important stuff, and make it really good.  Then review as much as possible to help students remember.  

When designing your teaching methods, start with objectives (e.g. improving students' TOEFL score), then pick your assessment (the TOEFL), then design the content (practicing things that are likely to be on the TOEFL).

When I teach, my challenge is getting rid of unimportant material so it does not distract students from the important information.