You need to decide who you are going to target as your participants. Your university probably has an ethical committee to scrutinise your research proposal and make sure you are not doing anything that could be unethical. Then you need to compose your questionnaire. What information do you want from your paricipants? Are you going to carry out some qualitative research, such as focus groups and interviews, or are you going to rely solely on the questionnaire and the data you might gain from With regard to your topic, and ideas about that, if you are interested in weight management, have you considered looking at eating disorders? Or the fluctuation of weight that occurs in females throughout their monthly cycle, when hormones are changing? This pattern of hormonal change might coincide with food cravings or aversion to food.
Carrying out primary research is very satisfying as you get to work with real people and see the subject at first hand. So I think your first set of decisions should be - who would you want to have as your participants, and what exactly would you want to find out from them?
Good luck with your dissertation. I hope youl do well.