No worries. Teaching conversational English is somewhat different from teaching grammar. Your objective as an EFL teacher is to get the students to use English. They get all the grammar training they need in school.
Focus on activities. How else are you going to get the students motivated to want to learn English and keep learning it?
The texts you'll be using will most likely explain the grammar points you need to know, and in cases where they don't provide help you can always ask a colleague or look up the answers online. If you don't know how to answer a student's grammar question, say so - tell the student it's an interesting question, the answer of which you'll look into and get back to them.
Teaching is not about being the authority of a subject; that is, just because you're a native speaker with a TESOL certificate in-hand doesn't mean you HAVE TO know the ins and outs of English grammar. You're being hired to teach conversation - modern day English usage -which is what you are an authority on, right?
Good luck and all the best to you.