The blade should have sharp, scalloped edges, which are ideal for slicing a loaf of bread without flattening it. A longer blade will grant you greater versatility for slicing larger loaves of bread. Ideally, your blade should be between 9-10 inches (22-25 cm) long. Sharpening serrated blades is a laborious, expensive process, so it’s best to replace your bread knives every few years rather than attempt to maintain a single blade. You can find high quality bread knives online or in home and cooking supply stores between $20-30. In order to get the best use out of your bread knife, it's recommended that you replace your knife every few years in order to ensure that your blade is still sharp and versatile.

Summary:
Buy a serrated knife. Opt for a longer blade. Choose an inexpensive bread knife. Replace your bread knife about once every five years.