If you've clicked on this blog, you must be captivated by the beauty of relativity, quantum mechanics, dark matter, or black holes. Now, you're eager to delve deeper into these subjects. Maybe this moment just happened recently or a few years ago. Regardless, you want to get paid to study nature and attempt to describe it using mathematics. As I enter my second year of my PhD in Physics, I thought of a few pieces of information that might be helpful to those with a desire to pursue a career in theoretical physics. This is by no means a comprehensive step by step guide, but rather a few tips on what classes might be worth taking before you go into grad school (for a more comprehensive guide refer to How to become a GOOD theoretical physicist by Gerard 't Hooft). Keep in mind, my interests lie in particle theory (there is a group theory joke here somewhere), so my recommendations will certainly be biased towards that.
If you are in high school, I would suggest taking AP classes if they are offered at your institution (I would recommend them over IB courses, given my experience, but that can be a whole other blog post), or self-studying for AP exams. I hate standardized testing with a passion, yet there is some merit to them in this case. If you take AP Calculus (AB and BC) as well as AP Physics C, you will be in a great position to start your undergraduate degree in physics. Getting exposed to calculus-based physics as early as possible will be incredibly useful. By the end of these classes, I recommend looking at a classical mechanics textbook, such as "Classical Mechanics" by John R. Taylor. If you find yourself struggling with this text, it is normal. There is a reason why physics is taught twice at the undergraduate level: you get a taste of all areas of physics in introductory courses, and then you take dedicated courses for each area.
At the undergraduate level, following the physics degree at your respective university will keep you more than busy, but I wanted to mention a few mathematics courses that would be incredibly helpful for your physics career. First, take a good linear algebra course. I would highly recommend a linear algebra course taught by a mathematics department rather than an engineering department. As a physicist, you care less about the mechanical aspect of computing matrices, and more about the abstract mathematical concepts. These concepts are essential for quantum mechanics, quantum field theory, and beyond. Linear algebra is indispensable. I would also recommend taking a complex analysis course and a group theory course, both of which will be incredibly useful in quantum field theory. Toss in a probability and statistics course for good measure.
I wish you all the best of luck! And I hope you don't lose sight of the moment that inspired you to pursue a degree in physics as you spend nights in the trenches, deriving the equations of motion from an awkward Lagrangian.