This is an incredibly broad question. First things first, you're going to want to learn about the make up of Operating Systems. Start with Windows (the most popular OS) and what makes it tick. Once you've got a good grasp for that, move onto Linux (and it's many, many different flavours).
Kali Linux is now going to become your best friend (https://www.kali.org/). Install that on a PC or Laptop and start to learn the tools. Read tutorials on what each tool does and how it help.
It's for ethical hacking and penetration testing, but ultimately you can use it for whatever you so desire.