Those numbers, at your age, are worrying, unless you have any other medical issues. You will have to start by improving your diet. Stop eating processed and packaged foods. Stop drinking cola, fruit juices, alcohol and quit smoking. Start with whole foods, lots of vegetables, fruits, milk, curds, protein. You will have to start including exercise in your daily routine, which can be walking, running, yoga or gym. Start small and build it up. Make sure you drink milk and also sunlight exposure is very important. Improving your bone density will take time, but with the above measures, it will help. Make sure there is no underlying medical problems by visiting a Physician.