Quantum Mechanics – for beginners
When starting out in Quantum Mechanics there are some areas of mathematics where you'll need some understanding in order to appreciate the theory. Roughly speaking, the big three are: Calculus, Linear Algebra, and Complex Numbers. Also, at the heart of the theory sits a partial differential equation, so some Differential Equations will be necessary for some derivations. Later, when going a bit beyond the introductory concepts, understanding of Fourier Analysis will pave your way.
What is Quantum Mechanics? In short, the best physical model for the smallest smallest possible things—like atoms for instance. Even if you don't feel like you completely grasp the mathematics, you can still have a look at this inspiring QM introduction made by 3Blue1Brown, in collaboration with Minute Physics.
I have had the fortune of being taught by PhD Babak Majidzadeh Garjani. His mathematical clarity, and mastery the lecture format, made the most unruly derivation comfortable to follow. Here you can find his (condensed) guide in mathematics requirements for introductory Quantum Mechanics.
Mathematics Companion to QM
Professor Allan Adams—a very charismatic teacher whose teaching style I fully enjoy—teaches the introductory course at MIT.
Video lectures, MIT Quantum Physics I
The second course in Quantum Mechanics at MIT is also great. Particularly the introduction to the Stern-Gerlach experiment, and spin in quantum systems. The maths of spin is complex enough that most lecturers spend their time with only that, and not so much connecting it to the rest of physics. But professor Barton Zwiebach has a thorough approach.
Video lectures, MIT Quantum Physics II
Looking Glass Universe is a YouTube channel made by a somewhat secretive creator. She seems to be doing this work on top of her studies, and she is doing it really well. For a pleasant and easy presentation of some core QM concepts, this is a great playlist.
Understanding QM with Looking Glass Universe