A couple of things that you should know before you sit with the PI:
1) Do a search on the recent publishings by the lab/PI and get a general idea of what the research is about or what (s)he is currently working on.
Note: you do not have to memorize any of the paper details however be able to have a stimulating conversation about the research
2) Look at the track record of the publishing from the lab i.e journals, posters, conferences, abstracts etc.
Also find out from past/current students as to how they are published/rewarded for the work that they did.
3) Be clear as to YOUR GOALS from the project. Make the PI aware of what your plans are and what is expected from the research from your side. (most are aware of the medical school process and can understand your side)
4) Discuss compensation/wages or hours of work in the lab (volunteer hours) and the schedule that you would be following.
If all of these issues are addressed before hand, they make for a smooth project overall.