As an instructor I have noticed that the trainees who have never stood in a cockpit before usually have issues trying to find their place in the cockpit. It is just too much information condensed in a very small place, and at different stages you'll need to discriminate what you really need to look at, and disregard the rest.
So, I would first grab a cockpit layout chart (the bigger the better), "sit inside it", and start building muscle memory reaching the different handles, controls, and buttons.
And second, I would try to make myself familiar with the PFD and ND: what info comes out in every line and in every corner, and where to find the basic flight parameters. For this I would go to youtube and just look at different videos about the PFD and ND layouts and functions. You will not understand most of it, but when the time comes to sit in the cockpit and start doing stuff you'll brain will be already used to a certain extent to discrminate what you need to look for at that moment.
And for the rest, as they say above, you lack the frame to understand how the airplane works and how each system interacts with each other, reading the FCOM on your own may get you confused at a later stage. Just be familiar on how to search for info in the manuals (iPad indexing, normal procedures, supplementary, etc...), and let the instructor guide you.
Enjoy it, I still remember my first type rating, twenty years ago. I have never experienced the same thrill again during training.