How can I lose some weight before summer?

I'm 15yr old girl, 5 ft and 145lbs. I want to lose some weight before the summer and then some during. I want to be able to be comfortable in a bathing around my boyfriends Mom, who insinuates my need for weight loss with all my clothes on. Don't get me wrong, I want to do it to be healthy and feel better about myself but I just don't know where to begin. Please help!