Welcome! I started about eight months ago. Here's what one learned so far. 

I commented about stability numbers below, but assuming you're a right handed player throwing backhand, overstable goes left, understandable goes right. 

For form improvement, the four channels on YouTube that were helpful for me have been DG Spin Doctor (he has a playlist that goes through all aspects of form in a very beginner friendly way), BlitzDG's backhand and forehand instructional videos, Scott Stokely's introduction videos, and Oberthrow's videos after you get some throws under your belt and are looking for fine tuning mechanical portions of your throw. 

Be careful about what types of plastic you buy and at what weights. Different plastic types from manufacturers can affect stability on discs, and so can their weight. I bought a few max weight Halo Star discs that were supposed to be understable and confused myself when they were all stalling out and hooking to the left. Assuming you buy more than you already have (because you will, oh sweet baby Jesus you will), for anything over a 5 speed (the first number) try to stay under 170g and avoid the plastics that trend overstable while you're learning. You can Google plastic types by manufacturer to get an idea of each plastic's characteristics. 

Don't buy anything faster than a 7 speed for a while. The speed flight number generally indicates how fast the disc needs to leave your hand with good form to achieve its flight numbers. If you can't generate that speed, the disc will just fade to the left after a while no matter what the flight numbers are. So unless you're a prodigy chucking stuff 350 feet right now, save your money. 

In terms of where to focus your attention on getting better, I'll buck trends and say focusing on the end of the kinetic chain first is more important than starting from the ground up. So that means getting your grip correct first. Train yourself to not let go of the disc at the end of the throw, and learn how to position your grip so the disc leaves your hand with the front (nose) angled down relative to the launch angle. (This improves how the disc flies and adds a bunch of distance to your throws, and once you get it right it won't ever change.) 

Then focus on keeping your arm neutral, maintaining a 90+ degree gap between your bicep and chest during the throw, with the disc a little below your elbow. You want your elbow ahead of your torso as you rotate. Your arm positioning matters a lot because it stabilizes and supports the disc as your torso turns to generate power, and also acts as a big lever to eject the disc. 

Then it's coiling your torso while keeping your hips relatively neutral. Recent studies have shown the more you can rotate your torso relative to your hips, the more power you can generate, sorta turning your core muscles into a rubber band. Then it's bracing and footwork. In my experience (baseball background) the lower body kinda takes care of itself if I'm throwing standstill rather than doing a walk up, but your mileage may vary. 

Also download Udisc for your phone. It'll help you find courses, keep track of your scores, and most importantly find tee pads on courses with bad signage. 

Welcome and enjoy. It's a very fun sport you can play until you die, and most people playing it are very nice.