How to get ready for a visit to a lab where I would like to do undergrad research?
You should treat it as an interview unless you get information telling you otherwise.  Yes you should bring a resume, but you may not need it.  You should read about the PI's research, lab, and team.  Usually there is some information on the national lab's website.  Typically undergraduate researchers are not expected to know how to do research, so you might be questioned about your motivations for learning to do research.  If your grades are not good you should be prepared to say what you are doing to improve them.