Some of the things you learn, either by experience or from good teachers:
1) Small light sources (on-camera, direct flash; direct sun) give harsh shadows.
2) Small light sources pointed down the camera axis (hot-shoe flash pointed forward, bright sun directly behind photographer) gives flat, harsh lighting.
3) Large light sources (lightboxes, bounce flash, overcast sky) give gentle or minimal shadows.
4) Light sources away from subject/camera axis give more definition, more 3D shape.
So
Start with using your flash in bounce mode. It uses up a lot of light, but it gives a much more pleasant look since the light source is large, and away from the subject/camera axis. Once youve mastered that, youll want to explore off-camera flash with diffusers, and then, multiple flashes.