You need to roughly know desktop security (especially memory corruption, C/C++ insecurities), web application security, mobile application and networking security. Those are (again, roughly) the domains.
If you want cybersecurity with web application testing (with overlap to mobile application testing), begin with The Web Application Hacker's Handbook. Best single treatise on the subject.
I also highly recommend the OSCP (Penetration Testing with Kali Linux) course and Cody Brocious' (daeken's) course called Breaker101, at breaker101.com.
If you want cybersecurity with web application testing (with overlap to mobile application testing), begin with The Web Application Hacker's Handbook. Best single treatise on the subject.
You should look through this entire thread:
https://www.reddit.com/r/netsec/wiki/start
I also highly recommend the OSCP (Penetration Testing with Kali Linux) course and Cody Brocious' (daeken's) course called Breaker101, at breaker101.com.
Good luck.