A farmers market is the easiest way to learn what is in season, and the stallholders are usually happy to explain.
Walk the whole market once before buying anything. Notice what appears on many stalls: that is what is at its best this week, and often the best value.
Ask how something is best cooked or stored. Bring your own bags and a little cash, since some small stalls do not take cards.
Buy a few things you would not normally choose. A new vegetable is a good excuse to try a new recipe.