Answer 1

There are a few things to learn:
-easiest is web security learn JavaScript,html,css, php, Python and sql and look into cross site scripting and sql injection after that you will have a good idea what to research next
-more complex is exploit developement and Reverse Engineering for that you need to know C/C++ and assembly and should look into buffer overflows.
-you should also look at the tools that come with kali Linux and try to unterstand networking(maybe do this first) but don’t fall in the trap of never learning how these tools work

From your profile I can see that you know java and python which is already a good basis. Especially python is very useful I think learning JavaScript would be best right now for you and try to get a big picture overview of how html works after that you can already start learning about cross site scripting and sql injection
By the way: the liveoverflow youtube channel is really good
Also look at this thread : https://www.sololearn.com/Discuss/1286284/?ref=app