I need to lose weight, please help?

I am a teenager and I need to loose weight for a cruise in december.I know i won't be able to enjoy myself if I am worried about how I look. I can't take and dietary suppliment and I only have access to the gym every two weeks but I can only go for 5 days during that time for 20 minutes and I need help. food is my comfort and it is hurting me more than helping me.