I also volunteered to do a job to get into grad school. My professor was studying dolphins and their anatomy. And I think he did that to weed out the people that are really serious from the people who weren't, because everybody studying marine mammals thought they were going to go out and work with Flipper. Don't be nervous or in a hurry about choosing something right away. Just follow something you enjoy.
Flexibility is good.
So is persistence. Sometimes things don't work out exactly the way you planned, but they usually work out for the best if you give it a try. Study science, but get a broad spectrum on the undergraduate level before you specialize. Be team players. You need to have the ability to work successfully with people and relate to people and understand people.