You have to do strength training in order to build lean body mass.You want to work out all your major muscle groups rather than just target areas. This includes quads, hamstrings, glutes, abs, arms, chest, etc. You will also want to make sure you are getting some cardio in each day.
First measure the percentage of your body fat using a body fat scale. Then strength train so that you build lean body fat. Consume fewer calories than you expend while increasing cardiovascular activity. Remember to keep a journal of your work out and calorie intake.