If I accomplish something impressive in my field of study before college, how would I show it to my dream college?

Depending on the application, there are different places where you could write about a particular accomplishment or achievement. Usually it would be an entry in your Activities List or your Awards/Honors list, with the possibility of dedicating additional space to it in the Additional Info section.

Some schools will also have the option to send optional supplemental materials (this is different from a normal writing supplement, which is usually required). E.g., MIT allows students to submit a "Maker Portfolio" which you can read about here: https://mitadmissions.org/apply/firstyear/portfolios-additional-material/. If you build something interesting that you'd like to share, you could do so through something like that if a school allows it.

Just like @jcdenton mentioned, you will have specific places on your application where you can put what you've accomplished. Using your examples, you would most likely put something related to a successful software company on your activities list while something like winning a programming competition would go on your honors/awards list.

While it definitely is helpful to have accomplished lots of impressive things before college, don't think that you need to only put those on your application. The more impressive an activity is, the better typically, but you don't need to have a leadership position in every activity or win an award at every competition for it to look good. Showing dedication to activities is also important. Since you're into coding, this might be helpful for you: https://blog.collegevine.com/extracurriculars-for-aspiring-computer-science-majors/. Good luck!