There are any books which tell how to find vulnerability on a system?
or maybe sites...

Throw a bootable kali linux on a flash drive and watch a few youtube tutorials on it, there're all kinds of pre-scripted stuff that can be run, the basics are a breeze. It's tough to master, but scary-easy to get started on.

@soohoonigan thank you.

i had some trouble last month with kali, so i gave up and i tryed to find something to read (for a theoretical kwonledge) in a first moment

Let me check my bookshelf tomorrow (or today, it's very late here and i should be asleep). I have a lot of books on those kinds of topics, i'll just have to search for the english ones that are worth their money.
If i dont reply just remind me again, because tomorrow/today will be a busy day.

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I did check and if you exclude the german books you're left with "hacking - the art of exploitation" (i really recommend it, it's old but it perfectly covers exploits at c-level), books from and with kevin mitnick (those don't cover computer hacking but they're totally worth the read) and the "hacking exposed"-series (you need preexisting knowledge for those books, e.g. you need to know html, js and http before you get into their web-hacking book).
For web resources i recommend owasp for a list of vulnerabilities still found in the wild and the security-stackexchange for answers to recent topics.

@YouAreAPIRate Thank You! You're such a good person