intrested in IB.

Was in your situation, so I'm glad I have some experience to share. Will get straight down to the point:

1. Finance or Closely-Related Clubs. You know the drill, it shows your dedication to your field of study.
2. Executive Positions in Student Clubs. It may not always work out, as you will (I assume) probably nominate yourself for elections, BUT you can start your own club. Top schools love initiative and leadership, so that's a great path to take. Doesn't have to be finance-realted, as long as you show that you have the skill of leadership, team management, etc.
3. Research. It usually happens during undergrad, but trying to do something similar would show that you would contribute to the university in the future by being involved in researches and helping push the academia forward.
4. Networking. It doesn't always come easy in high school, but still doable. Though, you can't put it on your application - it can help you out tremendously. Finding a valuable connection would offer you a unique insight into the admissions office, university's core values, etc. You can meet such people at fairs, conferences, and other types of events, which are dedicated to guiding students throughout their decision-making process. 
5. Internship. It's very hard to pull this one off, as you are still in high school; however, you never know what's out there for you. In my case - it worked out well for me. Absolutely worth the shot.
6. Compete. You don't have to take the top place (though that would be a killer), but showing that you are competing with other students on a regional/national level is something that admissions office loves a lot. You can compete in anything you'd like, I would say it's more about showing your courage and capabilities.
7. Try to Get Involved Early. By saying that, I mean the following: some schools have clubs/events that are open to everyone, even to those, who are still in high school. Attending those events or getting somehow involved would show that you actually understand the university's culture well, you've done your research, and that you would be a great fit for the uni's community.

Just to add a couple points, though that wouldn't be related directly to your question:

• Take the SAT. Though it's optional, that would boost your applications. It would give you an advantage over those applicants who don't submit one. Though not a guarantee of admission, but once you're going for a selective school - anything helps.
• Work on those personal statements as if you'll publish them. They are a heavy factor in college admissions. A good personal statement essay may turn the table around in your favor.

Mentioned it just in case you were wondering about that. 

Hope I could help and wish you best of luck! Keep us posted on how it will go.

P.S. If you have any other questions or if I've left something out - feel free to DM me, will always be happy to help.