Typically from personal experience these types of meetings are informal so no need to get overly dressed or prepare a large portfolio. Casual but nice clothing is definitely appropriate. It won't be a job interview but rather just a friendly discussion. If this is your first meeting about research, it will most likely just be a brief overview of what he does and potentially what a student would be doing. If you decide to do research with him then he most likely have future meetings in which he will discuss things more in depth, give papers to read to begin learning area of research and explain what is expected.
A professor is usually more than happy to discuss their research and take on someone whose interested and determined to conduct research if they have room. That being said, often times the professor would like that you have at least a general idea about what he or she does in their research and areas that interest you. You can expect a question such as, "What about the work I do stuck out for you and made you want to pursue this?" as I can recall being asked this more than once. It is also a good idea to have a few questions for them about their research, labs, etc. Again questions don't need to be an in depth inquiry on their research but just some light questions to get to know them and their work.
It's ok to be unknowledgable on the area of research as that's the point of research; to learn about the field and discover new concepts. The important thing is showing the desire to learn. Also you are an undergraduate so it's more expected that you don't have much experience in a research field.
All in all, from my personal experience it's a quick informal discussion with the professor to provide insight to you so you can decide if you want to conduct research with them. I've had nothing but pleasant experiences with this since professors love others interested in their research and I'm sure you will have the same experience! Good luck!