I’ve heard this question many times.
Let me tell you now that the Security field has many different realms and there is a ton to learn anyone who says they know everything about security is a flat out liar.
Think of it a number of applications and systems out there can you honestly think one person can know all of the systems inside and out?.
Sure you can configure a firewall or a UTM but can you point out a line of code in a web app that is causing a CSRF vulnerability?.
That is why I love this field there is always something to learn there is always a bug to find .
To begin your journey I would suggest a computer science/Programming background of some kind this helps you understand the logic of how a computer and network work.
Check out my friends at Engadget for an awesome post on how to get the basics of computer science without busting the bank and best of all online
Then move into learning systems I strongly suggest learning Linux as most of the security testing apps are built in and for Linux.
There are tons of ways to learn Linux both for free and Paid for check out this Link from Stack social on a course for Linux system administration course
After you got the basics of system administration I would also learn the windows operating systems.
Check out my friends at Cybrary https://www.cybrary.it/
Cybary has a massive collection of Free CyberSecurity training course I have personally take some of their courses and let me say I am impressed.
Check out my previous review of their course they have everything from basic security principles and networking to advanced forensics and hacking .
I am not a fan of their Linux training but everything out they have my check mark and thumbs up.
But the most crucial and important thing you can take all of the course and read all the books you want but there is a critical thing that’s sits with you.
This field changes like the wind you need to be interested you need to commit to learning and you need to enjoy it like everything else in life if you hate what you’re doing then find something that makes you happy.
I will post some other books and articles I have read in the learning resources section of my blog.
Let me know if you need any help or have any questions!