Film speed = shadow detail.
If you shoot ISO 400 film, photograph in normal contrast lighting conditions, and expose the same scene three times; once at EI 200, once at EI 400, and once at EI 800 (EI = Exposure Index). Don't change aperture, only exposure time (double for EI 200 and half for EI 800; this is called bracketing).
When you process the negatives, you judge shadow detail to see at which 'film speed' you had adequate shadow detail. It could be 200, 400, or 800, and you judge by printing the negatives, or making contact sheets. Use the same paper every time, for both contacts and prints. (If you scan it's a similar process, but I honestly don't know how to judge shadow detail with a scanner, so you're on your own if that's your process). Whatever your preferred film speed is (200, 400, or 800), for the results that you desire, that now becomes your 'normal' film speed (in normal contrast lighting). Exposure largely determines your shadow detail, which is why we're not focusing on developing time yet. This is half the part of "exposing for the shadows, and developing for the highlights".
After you establish your own film speed, you focus on the rest of the tone spectrum, and "develop for the highlights", which is why you expose a full roll at your chosen film speed, cutting the film strip into thirds, and adjusting your development time to get the highlights and mid-tones to look good.
With this approach of trial and error, you stand a much better chance at arriving at a film speed setting that works for you, and your personal film development time is also a bonus.
In time you will notice that high contrast lighting will require a different approach, and low contrast yet another. But finding a good solid method for getting the right speed out of your chosen emulsion, and a calibrated development time will save you tons of trouble in the darkroom when you print, or when you scan your treasures. It's the least amount of care we should give our films to begin to fully understand how everything works and hangs together. It's a good start.
If you change film, developer, or paper, you should do this process over again.