Where to start? What to do?
It can be very hard to learn Tekken once you start to get out of the button mashing phase and start to play the game from a more knowledge-based decision-making mindset.
There are just so many different mechanics to learn, so many moves and so many exceptions.
A good beginner resources list is set out in the Tekken Reddit Beginner Resources.
However, there really is a lot to take in, and I recommend you start with these two Massive Zug videos:
Then, move on to PeterYMao's Tekken Beginner Fundamentals playlist, and learn each mechanic one-by-one.
By one-by-one, I mean watching one or two videos, then going into Tekken to try them out in a real game. Then at the end of your session, review what worked and what didn't, and once you feel comfortable with the mechanics (not necessarily mastered, just enough to keep in mind), move on to the next video.
After you get the hang of basic fundamentals, you can extend your knowledge into knowing how to deal with character matchups.