Lemongrass tips -
Look for lemongrass that isn't overly dried out. The outer layers will be dry, but the inside should not be. It should smell very pleasant - citrusy with floral notes. It will keep well in the refrigerator for several weeks.
To use in herb paste - bash the end of the stalk with the back of a chef's knife. (Be careful not to cut yourself!) This releases the natural oils and the lemongrass will be more flavorful. Remove the tough outer stalks until you get to the tender center. Chop the center into small pieces and use for the herb paste.
To use lemongrass for soup - peel away the very outermost layers, bash the end with the backside of a chef's knife as directed above, and cut stalk into 3- to 4-inch pieces. Use as directed in recipe.
All of the spices and herbs should be available at any Asian market. The barramundi is available at Trader Joe's, Whole Foods, Stop n' Shop, Safeway, some warehouse stores, and other large grocery stores around the U.S.