How do I lose more weight?

I'm 5'7 120 lbs and anorexic everybody keeps telling me I look it but I don't know who to believe so far I've been living on atleast 100 calories a day maybe less and only drink monsters or water or just diet pepsi. I do sports and I don't know what to do. I also am a vegitarian so when I eat it wil just be a yogurt How do I lose more weight