How do I learn to secure different truck loads safely?

Loading a truck is already a skill, so give yourself credit for that. Securing every kind of load is a different thing, and it is normal to feel lost when the rules seem to change with every shipment. Nobody is born knowing this. It is learned one load at a time. The confusion usually comes from trying to memorize every case at once. You cannot. What you can learn is the handful of principles underneath them all: keep the weight low, block it from shifting, and use enough tie-downs for the load's weight. Most situations are just those basics applied in a new shape. Pick one load type that trips you up and master it this week. Ask an experienced driver to check your work and tell you why they would do it differently. Real feedback on a real load teaches faster than any manual. Do that a few times and the rest starts to click.