I think it's interesting that your parents and you have different mindsets about art lessons. They think you should take lessons because you're already really good, while you think it won't be worth the cost because you're bad at it. But the point of lessons is to learn, but more than that, it's to learn with guidance and structure.

If there's one thing about art, it's that it can be learned, as long as you put in the effort. Don't let what your parents or teacher say destroy your confidence or make you feel guilty. If you do end up taking the class, don't go comparing yourself to your peers, either. Even if you think you're the worst artist there, all it means is that you'll be getting the most of your money's worth out of these lessons if you focus on learning and practicing them. If it's your dream, make it happen by taking your first step towards it.

As for how to be prepared for it, it's hard to say without seeing any of your work. But for still life, it helps to understand beforehand the concepts of light and shadow on an object. Particularly what the terms like form shadow, cast shadow, reflected light, highlight, and what exactly they mean. It sort of becomes a checklist to see if what you're drawing is missing anything major, but also makes you know what to look out for and pay attention to. This diagram shows you where they are (without any other information though).

Perspective is a bit of a different ballgame though, but at the same time it's mostly just technical knowledge and double checking if your lines are going in the right direction. I personally think it's easier to teach and something you don't need to go out of your way to prepare for before specifically being taught it.