Students generally begin taking concentration courses in the beginning of the junior year.
Talk with professors about their areas of study, and with more advanced SES students about their choice of concentration. Start thinking about what you would like to be doing in 5 years, and in 10 years. Choose 2 or 3 possible areas for your concentration. Select some courses for each concentration, perhaps guided by the list below, and check out the prerequistes. Talk more seriously with professors, telling them about your goals and asking their advice about the best way to achieve those goals.