Agree completely with darrvao777. You won't figure it out in your first and second year, because you don't have enough exposure to the fields. Even if you think you don't want surgery, you might like it. I absolutely knew when I started med school that I wasn't going to be a surgeon, and that was really all I was sure of. When I started third year I was almost certain that I would be a neurologist, so I did my surgery rotation first to get it out of the way... and I loved it... skip ahead to now...I'm waiting for the match results so that I can find out which general surgery program I will be starting in the summer.
It's not just about what diseases you want to work with... it's about the culture of the field and the people you will interact with. Different specialties have different personalities, and you may not figure those out until third year. For example, I definitely have a surgeon personality, but I didn't realize this until I worked with surgeons.
There are questionnaires that are supposed to help you figure out what you want to do, but I don't think they are too reliable. I always get wildly different results on different tests, or I'll get ridiculous results (like #1 will be cardiothoracic surgery and #2 will be primary care - that's absurd). Don't count on those to be accurate, although they can be fun to do.