As noted in the other answers, you will probably be fine with Mac OS, Windows or a flavor of Linux, but in the past two companies I've worked for, most of the developers were on MacOS or Linux - especially of the company's tech stack was open source or Java-based. If you have interest in .Net programming, Windows would be a good choice. Windows went through a phase where it was considered less secure, less stable and less "cool" than the other operating systems. That has largely passed now. Being competent in multiple operating systems (with maybe one of them where you are particularly deep) would be even better.
Ted recommends the following next steps:
- Reach out to the CS departments at the colleges you are interested in and ask about what OS's they support
- Sometimes cost is an issue - Macs can be more expensive
- Talk to students in CS programs you know.
- Think about what OS you really want to get proficient with as you look forward to your career.