Bring a copy of your resume, any sort of Certification you've received (ACSM, ISSA, etc.), and know your availability. Understand that some facilities can move the training schedule around as long as it isn't drastic, most keep their trainer hours the exact same which may become an issue if you can't work around it.
As for preparation, go over the material you studied for the certification exam. Then go through a couple practice interviews/scenarios with a friend or you can find a few case studies online where you are presented with a client who has certain goals and limitations. Go through the steps you went through with the certification exam and it will show that you have an understanding in how to handle clients and how to engage with any scenario in which may present itself.
Make sure you look professional, its not an office job so you don't have to wear anything nicer than a polo and khakis. I went in with joggers and an athletic shirt, just to give the employer an idea of how I play the role.
Differentiating yourself as someone who practices what they preach, rather than teaching and leading a different lifestyle. I also came in with a backstory and my reason for entering the fitness industry.
If you don't have a certification, they may work with you as long as you are either working towards it or are currently enrolled in school focusing on a major related to training. I was both working for a certification and was enrolled in Kinesiology program at my university.