My BMI is about 28 and i need to lose weight. Help please?

Ok as you know it is April 22th and summer is coming. And i am a little overweight. My BMI is about 28 and i need to lose weight before summer comes (about June 15) that i am starting going to the beach. I am 15 years old and 1.73m Height and about 85kg of Weight(not sure our scale is broken). So how many kgs can i lose by then if i try hard?And i need some methods to help not pills and stuff like that but more a healthy diet or something like that. So my main question is how many kg i need to lose and if i can before summer. Thanks in advance!