I am 18 and has to lose as weight as possible in less then a month?

Think long term: If you'd like to lose weight, in less than a month, get started immediately. Start by being sure that you're medically cleared/safe for an exercise/diet program. Then, increase activity and improve diet, especially by keeping your portions smaller. But, rapid weight loss tends to be risky and unsustainable in most cases. So, a long-term health plan is necessary for sustainable fitness.