Reddit mentions of Introduction to Radiological Physics and Radiation Dosimetry

I agree with everything said in this thread, but to add some things:

I strongly recommend a good particle physics course. If you can think about the basic things in our field automatically in terms of Feynman diagrams and you know the rules of certain force interactions (everything but gravity being important, with EM and Weak force interactions slightly more important than strong in our field), I've found that helps me to understand the empirical/applied stuff a lot better.

I would seek to shadow as many medical physicists from as many different specialties as possible. It'll let you decide if you really want to be/what specifically you want to do in our field, as well as look good on any resume. I'd shoot for at least one shadowing session of clinical physicists in radiotherapy, nuclear medicine, and diagnostic imaging each, as well as a health physicist, and an undergrad internship at a company like Varian or Elekta (look to the AAPM for undergrad opportunities/funding). Some physicists appreciate teaching people while others don't, so don't give up if you're turned down on a cold call, but don't cold call the same person twice.

As a bonus, read Kahn, Attix, and Hall, if you have time. For Nuclear Med, I'd recommend Cherry, and for Diagnostic Imaging I'd recommend Bushburg. Definitely start with Kahn as it is the easiest to digest IMO. I'm sure others have book recommendations as well. Though, I wouldn't bother until you've at least taken your entry level classical mechanics and E&M courses, even then a lot wont click until you've taken Quantum 1.

Edit: oh also experience in programming will help. Matlab and/or python seem to be base in our field, though C++ cant hurt.