You should be business casual. Tie and dress shirt at least. You might be asked a variety of questions, depending on how intense your interviewer wants to be. It generally will be "Why do you want to do research?" If this is a basic science lab, you want to avoid mentioning that you're "premed". Being interested in the sciences is enough. Some interviewers might ask what you know about the research that the lab is doing and might test to see if you've done your homework. Read up on what the lab does in preparation. I like to read a couple papers published by the lab to get a feel for the type of research that they are doing.