First of all, hi there.
Being quite new to this too, I can tell you, you can learn fast and you've come to the right forum.
The most important thing is not to get math right, but to have fun playing.
Personally, I would first play a career with only low/medium difficulty to learn about the simulated environment: what's it like "at sea"? How do convoys work? How do I approach them? Where do I find targets? How do escorts "think" and react? etc. Play with map contacts on and TDC (the targeting "computer") on automatic and send a couple of ships to the bottom.
Then you'll understand the videos so much better, because you have context.
The TDC is complex, but not really difficult
and it mainly does two things for you:
a) I you get at least two measurements on a target ship, it can calculate speed for you (very useful, but not 100% accurate, so get as many measurements as you can - well, several, not unlimited),
b) The position keeper will predict enemy movement for you (so long as the target does not change course), giving you firing solutions for your torpedoes.
Beyond that there are Dick O'Kane, sonar, radar, vectoring and half a bazillion other creative ways that push the system far beyond what it was designed for, which will allow you faster and very accurate attacks under diverse conditions.
Everybody (except maybe the actual submariners that learned everything in real-life) learned it step by step, so just take your time and it will come all by itself (mostly).
Just my two cents, the experts here will put it more accurately.