What should I do since I am fat and have to be in a bikini with friends in public?

Okay so I have to go to the beach in about 10 days. I am not bikini body ready but the only bathing suits I own are bikinis, I use to be skinny that's why...until this winter when I just sat around on my laptop. Now I feel fat, and I think I look fat even though my friends think I look fine. I don't wear tight shirts so they don't know that i'm not skinny. Anyway I look in the mirror and feel like a hamster or something. I've been eating less + healthier. & I know i'll have to start exercising more than I do. But in the meantime, I HAVE to wear a bikini and go to the beach with my friends. I don't want them to see me like this and let alone let all the other beachbums see me looking like this ! My friend's are all very skinny. Like underweight skinny sooo even if i was in shape, i would probably still look fat while standing next to them. I don't know what to do, can i maybe get some girl advice from someone who's been through this sorta situation before ? xx. And no mean answers. Please.