Although you certainly can learn alot from others experience, you can also learn from your own. Try lots of different flies (or lures and baits if you're spinfishing). Pay close attention to what works and under what conditions. Look along the banks and in the water to see what natural baits are presenting themselves to the fish. Are grasshoppers jumping all around you as you walk to the river or lake. Are there lots of crawdads or minnows in the water. Are there insects hovering over or in the water. See whats currently available to the fish and try a similar bait or fly. If you can't tell what the fish might be feeding on then try different baits and techniques until hopefully you find something that works. Try floating your bait with the current. Try different depths. Try retrieving your fly with different speeds and cadence. Try different types of water. Sometimes where fast water meets slow is good. Sometimes the fish are in the faster shallow water. All the while you're experimenting pay close attention to whats working and what is not. After some time you will start noticing patterns to what works and when and under what conditions. And as the years go on you will begin to become a master.
Of course there are lots of fishermen with years of experience on this forum who will be glad to offer advice and answer your questions.