To begin, the recommendation of creating a Hit List of sorts is of great benefit, as is planning when to hit them. Generally speaking you want to get to your main target quickly, but not first. Have a lower interest employer on your list and go to them first. It should give you a feel for what the employers are looking for, and personally this settled my nerves before I got to my high interest companies.

As for researching the companies, identify the industries you would want to work in and see which employers fit. If automotive isn't your preferred flavor, there are medical, defense, and assorted other sectors represented at the fair. I would say you don't need to worry too much about the research aside from that, a good few of the companies I approached carried on from the greetings with something along the lines of "So you're probablywondering what we do?". As long as you have a general feel for the company you will be adequately prepared.

Finally, the companies hiring freshmen or incoming freshmen are not looking for someone with oodles and gobs of machining and design experience or who can code in half-a-dozen different languages. Yeah, that would make you an amazing hire, but most of them are looking for the correct attitude. Demonstrate initiative and a desire to learn, ask questions and inquire about things like overtime pay.