It is highly unlikely that you can't lose weight for medical or genetic reasons. Look for the more likely answers first - for example, that you need to make changes to your diet to incorporate healthier, lower calorie choices. Get hold of a nutritional guide; they are available on the internet and in libraries and in bookshops.
Familiarise yourself with different food groups and how much you should be eating of each; find out how many calories are in common foods - you might be surprised. Look at the nutritional value of different foods as well as the calorie content; find out about official advice about nutrition and how much you should be eating of certain types of food. Don't become too hung up on it - it's important to have a balance and to enjoy your food. But develop your knowledge of food and nutrition so that you can make informed choices.
Before starting any weight loss or exercise plan you must consult a medical professional.
For further weight loss ideas and advice,
visit the substantial resources section of this site
Weight Loss Resources