Don't stress too much over feeling overwhelmed - that is pretty natural when first jumping into any fighting game, much less going from 2D -> 3D rules (especially one like Tekken with big movelists for each character). As to your rule questions, the main things to remember are that all movement commands cancel into other movement commands, and movement commands cancel into attacks. Being able to cancel an attack into movement (like being able to back out of an attack before it comes out) is character-specific stuff and will be in the movelist; as a standard rule you generally cannot.
I'd probably focus on trying to get comfortable with the movement of Tekken, and once you figure out which character appeals to you the most, start trying to figure out that character's sort of "key" moves. Don't try and learn how to apply every single attack, because there is a nearly infinite amount of situations in Tekken and a lot of those moves aren't that useful outside of certain ones. Knowing all that shit - like how to adjust your combo if you are coming at a wall at a weird angle - is stuff you can only learn over time.
Just look for some basic stuff that you can put together to get a general gameplan going using some of that character's better moves. You can look at various character tutorials, but this link is pretty useful in this context:
https://drunkardshade.com/2017/05/27/tekken-7-top-15-moves-for-all-characters/
Like, I haven't played Tekken for 20 years, so I kind of feel like I'm in the same boat almost. So from the jump I'm trying to just grab like ~8-10 moves out of Paul's moveset that are good and give me a wide enough set of tools that I can focus on learning how to move and still get some damage out of as many situations as I can recognize (which isn't many right this moment)
For the most part I feel like Tekken is almost entirely movement and knowledge based, even moreso than 2D fighters. Let yourself learn all of the knowledge part over time - including your own character's moveset - and focus on learning how to move around, otherwise it'll feel like you never seem to have the opportunity to use any of the moves or combos you bothered learning because your opponent will always be in the wrong place at the wrong time.