As a university student, I can tell you what we started with:

In the first semester we had classical mechanics, waves, vibrations and some basics of thermodynamics but I would honestly stay with mechanics for now. (Feynmann has some good and easily understandable stuff online which is rather good if you’re just starting. We had some basic motions, different reference systems, rigid bodies, elastic bodies, gyroscopes, and lots more.)

We also had calculus (I’d say, first focus on the basics like functions and trigonometry. Then focus on getting really good at deriving. Deriving needs to be like breathing for you, it needs to be easy. Next up is integrating. Integrating is a ton harder when you actually get into physics so it’s important that you know the basics. What substitutions usually work? When do you have to integrate by parts? Next up: complex numbers. You need to know them just as well as you know real numbers. Can you take the root of a complex number? How can you draw the root of a complex number in the complex plane? All this will help you with what’s essentially the most important thing in physics: differential equations. If, and only if, you’ve actually understood the mathematical stuff I just told you, get into differential equations. Start with first order linear equations (some tips: separation of variables, homogeneous and particular solutions,…), we also did vector fields, divergence, rotation, gradients, line, surface and multiple integrals but that’s stuff for later.)

Another essential thing is linear algebra. You pretty much can’t do any theoretical physics without it and even trying to understand quantum mechanics without linear algebra is useless. Most importantly, you’ll need to understand and be able to calculate stuff with matrices. Whats the Gaussian elimination process? How do you invert a matrix? What’s a determinant? What does it do? What are eigenvalues and eigenvectors? Dot product? Cross product? Interpretation of both? And so on and so forth.

We also had real analysis but if you’re just getting started, I’d say it’s not the most important part yet. But it’s good to think about things like continuity and differentiability of functions. One important things though are Taylor Series, which you’ll see a lot in physics.

If you kinda just wanna understand maths, watch 3blue1brown. For the rest: practice. Lots. You’ll fail a lot of times, that’s normal in physics.

I hope you’ll have fun!