After your check up, if everything is ok get a plan. For example they have charts that show how much you'd need to work out for what you eat. Like you need to run 2 hours to burn off a hamburger.
Once you learn these things remember all calories need to be burned off by activity.
Most sensible eating plans inc the highest quantity of raw vegetables. Some fruit and nuts. Cook lean protein like fish in olive oil rather than soy oil.
Pick an activity. At school. If you can join a gym or participate in community classes which are less expensive.
This is something permanent. You need to change your habits to healthy habits. It's doable but takes time and effort. Plan your meals ahead of time. Make sure you exercise everyday for a minimum of 30 min. An hour would be even better. You can do it. Pick your plan and stick to it. Maxy
Before going to a diet, get a physical by a physician, and make sure they perform tests such as a TSH (Thyroid hormone), and testosterone There are many reasons for obesity.
If there is no hormal issue, you can begin a diet program.