You want to learn to speak perfect English by tomorrow? You're expected to speak a lot in interviews, so if you have less than perfect grammar, that is going to come across in the interview. The important thing is to be confident, because you will not be judged on the suitability for the job based on your grammar, unless it is directly relevant to the role. A good idea would be to think of possible questions you might be asked, and think about how you might answer them. I wouldn't go so far as to prepare any answers though, because it will sound very artificial. Remember, an interview is just a conversation. You already have the skills to do the job, or they wouldn't offer you the interview. The interview is more about building up a rapport with the interviewer, and proving that you have the knowledge and/or experience to do the job, and you won't be able to do either if you're worrying about your grammar. Don't worry about your grammar, because there's nothing you can do to improve it by tomorrow. Focus on what you might be asked instead.
Not a teacher.