The most important thing in photography is the interaction of the three components. Ultimately, the brightness of your image depends on the combination of shutter speed, aperture and ISO. In the end, however, you always have to act according to the situation. It's best to try things out a little. Try to capture fast or slower movements. Sometimes try to bring sharpness, sometimes blurriness into the picture.
If you want to photograph (fast) sports movements, for example, you need to choose a faster shutter speed. It is also easier to capture the movements if you set the aperture so that the entire image is sharp. You can then use the ISO to bring the missing brightness into the picture. For darker scenes (sunset or night photography), on the other hand, the shutter speed needs to be rather slow (e.g. 1/60, ideally using a tripod), as this also creates a certain brightness in the image. If you also want to have a specific subject in the foreground, you can open the aperture accordingly to bring blur into the picture. You can adjust the rest with the ISO, taking particular care to avoid image noise. If the image is too dark, you might want to experiment a little with the shutter speed and aperture to brighten it up!
You should also always make sure that you set the focus correctly. Because if the background is in focus but the model is out of focus, then no matter how good the other settings are, the picture will still be nothing (unless the effect was chosen on purpose). You can set the focus to automatic or adjust it manually. That's up to you. Autofocus is usually easier, but sometimes you can work more flexibly with manual focus.