Before you go, you should figure out which colleges will be there (you can usually find a list online), and write down the names of a few schools that interest you. Come up with some questions that you want to ask, and write those down too so that you can easily reference them at the fair. It's also a good idea to bring a pen and paper so that you can write down any information you get from the college representatives and a backpack to carry any informational brochures you happen to pick up.
When you're at the fair, don't be afraid to ask lots of questions. The more information you get out of it, the better. Take some time to write down what you learn after you visit each booth, and give your email address to colleges if you're interested in getting updates from them in the future (if your email is currently email@example.com, now is a good time to update it to something more professional - first initial and last name is a solid choice). Take any brochures or other literature that's offered by colleges so that you can look more in-depth at what they offer later.
There may also be a few schools that you didn't write down initially that spark your interest at the event. Keep an open mind - you don't have to stick to your list, and you might find a school that's a great fit for you that you otherwise wouldn't have considered.
When you get back from the fair, use the information you've gathered to narrow down your college choices and conduct additional research. If you know you really liked a few of the colleges, you can start planning some campus visits!
Answer adapted from this article.