Which branch of the military should I join to be an IT (information technology) officer in the active reserve?

You can join the National Guard or Reserves but there is no guarantee you will be able to join the Active Reserves.

You can join active duty but there is no guarantee that you will be branched into Signal.

If you have enough certs and experience you may be able to direct Commission into Cyber but who knows if that will still be an option by the time you finish your degree

IMO talk to USAF and USSF recruiters first. Air Force and Space Force missions lend themselves to large use of all types of IT. That said all the Services use IT extensively and need IT specialists and professionals. Talk the other recruiters also and take the offer that most closely matches your personal life goals.

Marcos Gavis Join any of them and You'll be working for the Same Boss, All cyber falls under the NSA. Of course I'm Preferential with the Navy, 10th Fleet, Information Warfare Command. Cyber in a Navy Uniform. A Cryptologic Technician.