One of the best introductions to the field is going through overthewire’s bandit vulnerability games. https://overthewire.org/wargames/bandit/

They have 30+ levels where you ssh into a server and attempt to find some type of vulnerability. They start out very easy and get tough quick. It’s very eye opening to see the types of exploits that exist.

They also have a set of challenges aimed at serverside web security. http://overthewire.org/wargames/natas/ I went through the web challenges last year and they helped a ton in my web dev roles.