You will most likely narrow it down quite naturally as you take more classes. As you take higher and higher level courses in those subjects you will get a better idea of what kind of time, effort, and skills are expected for those particular subjects and it will start to become clear enough that you can realize "This just doesn't suit me" or "I'm pretty good at it, but it's not interesting" or "It's interesting but I'm not very good at it. Am I interested enough to fight the uphill battle?" and hopefully at some point "This is interesting and I'm quite good at it."
I started out with a similarly broad set of interests (math, physics, computer science, biology, engineering). For instance,
Starting college:
Math is interesting and I'm pretty good at it
I'm good at Biology but not very interested
I am good at programming if I'm dedicated enough, but I'm just not interested enough to take a lot of programming courses. Theoretical computer science is interesting but it is practically mathematics anyway.
I'm not good at physics, but it is very interesting.
I'm similarly not good at engineering and it is not very interesting
3rd year of college:
First semester:
Time to pick a major. Either math or physics. Take a chance and go with physics.
>Bad idea, not that interested after all (after taking three higher level physics courses)
Second semester (now):
So switch to math
>Much better
Applied or pure? Continuous or discrete?
>Liked intro to DE's a couple years back.Taking PDE's now, hate it.
>Taking advanced calculus, like some but not all. Hard for me to visualize things.
>Proofs, not bad, I get some enjoyment out of
writing proofs.
>Liked discrete math and did well, looking forward to graph theory/combinatorics next year.
>always thought better algebraically then geometrically or analytically.
So probably not applied math, probably not differential geometry.
Logic? Not enough experience.
Possibly some discrete math area and/or algebra. But don't forget continuous completely.
That's just the process I've been going over as I become exposed to more advanced topics.