A few #fosdem advices for newcomers: wear a mask (it's very crowded, it's cold, you're in the middle of winter, people will be sick and there will be a flu cluster).
Wear warm clothes and preferably layered clothes: entering and leaving buildings all day long, you'll enjoy being able to open a zipper in the overly heated buildings and going out in the cold. Bring a rain coat / an umbrella as it tends to rain quite often.
Arrive in advance, otherwise you might miss your most-wanted talk because the room was full.