Seriously...it's like places to race cars. No one tells really or the good sources dry up.
Back in the days right prior to the Great Hacking war before hacking made you part of America's most wanted, a dude called our house (It was around late 80's maybe...) and asked this question.
He was able to trace our connection and then access phone records. We wanted to learn more. My brother and I gave him a list of books/gfiles to read (as a joke, ones in our bookshelves on UNIX, x.25 networks, etc). Told him to call back when he was done.
Dude read them all and knew the crap hands down. Later became pretty skilled at figuring out some really cool stuff. When on to become a CTO of one of the original private ISP's, multi millionaire by 25.
Today there is really 99% script kiddies. No one really does great hacks anymore...it's hard to play though when you are looking at fed time for getting caught.
I pulled out of it right after both the FBI and AT&T had a family sit down with us.
You can search for cDc, Phrack, 2600 (which may have meetings in your city), etc.
You start outside the rabbit hole and work your way in.
Our biggest profit was a local phone bridge that was not on the record so no billing/auditing took place. It was a local call for us was where the N word comment was dropped to Corrupt by Eric Bloodaxe who thought he was SN.
In our day the big places were tchh, altgiers, lutzifer, QSD...
Hacking is like Engineering or Medicine...you need to pick a niche to focus on.
Figure out something really good and get hired for big $$$.