It doesn't matter that you are not great at the moment, if you were, you wouldn't have to take lessons, right? =)
It is an opinion based question, but I like it, so I try to give you may impressions.
One thing is to prepare one or two songs, maybe two quite different songs, which represent what you want to sing in the future. If you want to sing Rock/ Metal, you wouldn't prepare a hip hop song, unless you want to sing that in the future, so that should be clear anyway.
Besides that, there is no really preparation needed. I assume you already had a trial? Most schools in my country give you an hour for free to get to know the teacher and his concepts, also there he gets to know you.
If not, it is really important that you get to know the teacher and he gets a feeling for your level. If he is a good teacher, than you don't have to worry about anything, he sets the course of your lessons and with more experience you will encounter problems or things you want to solve/ try.
Bring your songtext and chords/ sheets for your songs, that way he can accompany you on a piano or something like this, makes things easier to practice.
But if you are a beginner, you will propably start with breathing, posture, facial expressions, tongue position and basic exercises.
Most important thing for me is that my teacher knows his stuff and that I like him on a personal level. You don't have to become best friends, but if you feel uncomfortable in his presence, lessons will not go that smooth and won't be fun.
Also, if he is too fast, or is expecting too much from you, tell him. He should know what he can expect from you (after a few lessons) but if he doesn't notice you are unable to cope with it, tell him.
One important questions is to set your expectations. A teacher won't make you a great singer automatically, they can only show you how to get better. So, there is still effort involved. They can show you how to improve aspects or your technique and everything, but they can't rework your voice completely. Also, there are a lot of things to be learned which are important for different styles. So try to get an idea of what you want to achieve and then work on the points needed to get there.
The best feeling you can have is during the lesson, that you think "Wow, I didn't know I could do that", then you know that you are on the right track.
But you also have to prepare for phases where you think you are advancing too slow. Don't get upset and immediately change the teacher. Speak with him. That may be intended because you maybe have some deficits you have to work out in order to advance further. But a teacher should be able to tell you what is wrong and why you are not advancing further. Also it takes time. Prepare to take your lessons for at least a few years if you are serious with becoming a good singer. Because some things have to become subconscious in order to concentrate on other things while singing. I had my first concert and I couldn't take lessons two months earlier. I had some problems on that gig, and as I found out later, I made a lot of mistakes because I didn't remember some things from the lessons, they were not subconsious and I did things wrong, which resulted in a bad performance. So, prepare to take your time. =)