Read the lab website and a couple abstracts enough to have a general idea what the lab is studying and why you're interested in it.
If you are interested in having a long term project resulting in an abstract or publication, mention that in the beginning. If not, no need to.
Bring a notepad, paper, and your CV. The PI might show you the lab and give some details about what kind of things undergrads have done/can do so take thorough notes.
Also, when comparing labs, focus on a few things:
What is the quality of research coming out of the lab?
How responsive and engaged is the PI to your questions/will they make a good mentor?
Will this PI kill you if you make a mistake? (You will make mistakes starting out- everyone does haha, it's a part of research.)
Do the people in the lab appear miserable or excited about their research?
Is there the opportunity for a long term project?
How many people are in the lab? 15 students and 1 PI means you will never see that PI haha. If there are a lot of people but a good Postdoc:Student ratio you could be ok.
Don't be afraid to ask students in the lab how their projects are going and how often they get to meet with the PI. If the answer is they're not and never, then you figured out which lab not to join haha.