There are dozens of grammar websites which range from basic to advanced. I would recommend you get your hands on a copy of Raymond Murphy's English Grammar in Use and do some serious cramming.
Find out exactly which grammar areas you're going to need to teach and concentrate on them first.
In the meantime, if you're struggling with a particular issue or aspect you can ask us to help, giving us some example sentences and what you don't understand about them.
I have moved this thread to the "Teaching English" section as that is where teachers help each other.
Please note that although you said you learned to write English while living in the USA, you failed to capitalise "English" throughout your first post. Remember that the you're going to have to teach correct written English with all its rules of punctuation and capitalisation as well as grammar.
Remember - correct capitalisation, punctuation and spacing make posts much easier to read.