There are many reasons grass can turn brown. First, are you watering enough? Dig into the soil a bit and see if it's dry. Often, a light watering barely penetrates the thatch. Make sure you water at least a half inch a week.
It's also possible that you have disease or pest problem. This is difficult to diagnose without seeing it. But in general, pests and diseases cause grass to turn brown in patches or irregular shapes. Drought, however, tends to cause grass to turn brown more uniformly over most of the lawn.
If you think it's a pest or disease, take a sample into a reputable garden center or extension office or master gardener office for a diagnosis.