How often should a 14 year old workout?

i'm a girl. i'm native american, black, and white. i'm bigger than other girls. i'm not fat and i'm not bone skinny. i'm about 180 and 5.6 feet tall. school starts in 2 more months and i want tto be comfortable with myself. my dad bought an elliptical. i have muscular legs. i'm not sure how to start how to lose weight. can you write out a diet plan and workout plan. i'm done growing. i haven't grown in like a year. please help me!!