Welcome to the community!
Generally speaking, everyone learns differently, but I’ll tell you how I went about learning to play fighting games.
For me, I also played a lot of fighting games in the '90’s, mostly the Mortal Kombat franchise, though I was younger and played them casually. The first fighting game I sat down and learned to play seriously was Mortal Kombat (2011) on my Xbox 360.
Basically what I did was played through the Tutorial to get a basic understanding of the fundamentals for the game. I next played through the Story Mode for some simple nostalgia, and to experience playing as a bunch of characters and the different styles of characters available.
I found I liked aggressive characters, and I ultimately decided on Mileena as my primary main. I then went into the Training Room. There, I poured over her move list, and simply did her normal attacks and such, getting a feel for the speed and range of them. I then did the same with her Special Moves, getting a good idea for their properties.
After that I looked at the basic strings for her and simply did them until I could do them without thinking. I then moved on to see what Special Moves I could Cancel into to start doing better combos.
After I was comfortable with that, I went Online to look at Combos others had come up with; not the super advanced ones, but better than I came up with on my own so I could start saving Metre better (and not using X-Ray attacks), and thus better learn Metre Management while still dishing out damage.
At each phase, I’d practice what I learned against the AI. Once I could consistently and comfortably beat the Hard AI, that’s when I felt ready to start playing against real people.
For Killer Instinct, I recommend doing the game’s Tutorial as well, practicing in the Dojo, and also against the AI to apply what you’ve learned in a simulated multiplayer situation. Don’t rush your learning, take the time to truly understand and get comfortable with how the game, and your character, plays.
I also strongly recommend Infil’s guide here; as it’s detailed, easy to understand, and filled with exceptional information. It also break’s all the characters down and can help you choose one if you’re not sure, based on the style of each character.
Hope this helps!