How do i stop eating? And how long must i do it to lose 20 pounds?

Its summer and i still havnt been to the beach but i wana go before the summers over. And this year i wana be able to wear a bikin! Im 16, 5"5 and 150 pounds. But i wont wear it if im not at least 130. I joined a gym a few days ago and im tryna learn portion control (obviously its not working which is why im desperate and hav decided to go to extremes). I knw its unhealthy dont lecture me. Only answer if you have a good one. Thanks (: