Congrats on landing your first interview! That's the hard part. The easy part is the interview. Don't stress out about it. You should dress nicely - in a suit, preferably, that's pressed and fits well. Bring extra copies of your resume and if you have samples of any work you've done in the past (lesson plans, etc) bring those as well. You'll never know when you will be kicking yourself for not bringing an example of that great project you completed that is just too hard to explain in words.
Prepare for the basic interview questions - like - name a time you failed in the past, why did you fail and what did you learn from it? Don't be overly cheesy (that seems forced) but always spin things positive. Never focus on the negative. If you spurt out a negative because of nerves, that's ok. Just come back in and say "but I'm also... xy and z" and I did "abc" which everyone loves me for.
Best of luck!