I weigh 170 and 5'1, iam 20 how can II lose weight fast?

Weight Loss. Weight loss should always be a gradual process -- aim for 1-2 lbs. Per week through diet and exercise -- this is sustainable. Rapid weight loss usually does not work, and leads to a weight "yo-yo" with rapid gains and losses.
Food volume . Don't eat so much. Eat 4 times a day, no more than would fit in your hands cupped together. Of course, choose reasonable types of foods. Then, if you're so incline, increasing activity wouldn't hurt.
Diet and Exercise. Several things to consider. Look carefully and honestly at your diet. Cut out all unnecessary calories such as sweets, high calorie snacks, sodas (unless diet), and fats. Consider a vegetarian diet for a month or so. Begin an exercise program of some sort, at least walking. Hopefully you can lose 2-3 pounds a week. It is all a math problem. Calories in versus calories burned up! good luck!