How to hack yourself to prevent other from hacking you?

I am learning to become a website and software developer. I want to be prepared for eventual future hacks on my apps and programs. I want to learn about ethical hacking so I can ensure the best security of my products.

I was looking for free beginner hacking PDFs to read, but there are too many of them. Do you guys have any free PDF book recommendations on practical beginner hacking for websites and software?