Hi, I am a 20 yr old female and I was wondering what good dietary supplements are there to take?

I've been screened for hyperthyroidism and I do not have it but I eat right and exercise but I can't seem to lose weight. I have weeks where I can lose up to 5lbs and then other weeks I gain it all back. I can't seem to lose any weight. I have a one year old and after being pregnant, my body changed dramatically. I don't know what else to do. I go to the gym, I've been on weightwatchers for months and whatever I lose, I gain back and I just don't understand why. I need advice, please.