Losing weight

I’m 16 years old and I weigh 207 pounds. I’m not happy at all with my weight. I see all these girls with thin wrists that I wish I had. My boyfriend told me that I just need to set my mind to it and do it. I have tried everything and one time I was losing a lot but I have no clue what I was doing right after I stopped. Please help give me some ideas oh what to do and how to lose the weight.