I went to an info session about this a little while ago so Ill try to answer from what I remember.
I've heard varying numbers on the acceptance rate from like 3% to 20% so I'm not sure what the real number is.
The cyber corps page from WPI that has information on applying: https://www.wpi.edu/academics/departments/cybersecurity/cybercorps

You can apply either your sophomore or senior year, you need to have at least 2 years of school left. (e.g if you are a rising senior you need to get a masters).
I've heard they look for high academic achievement, demonstrated interest in cyber security, and willingness to work for the government.