Sitting or resting for long periods will make your joints stiff and your muscles weak. Gentle walks and stretching excercises can help. Swimming is an excellent sport, and can help you keep mobile without putting a stress on your joints. If you are overweight, you should try very hard to lose weight. There is no evidence that special diets are helpful, and you should have a healthy diet which includes fresh fruit, fresh vegetables, and dairy products.
Some people find taking cod liver oil helpful.
You may find getting out of a low seat is difficult, and many modern seats are too low and soft. You should choose a comfortably padded, upright chair, with a firm back and arm rests. If you have arthritis of the knees, hips or ankles, wearing cushioned shoes can help. Choose shoes with a good grip, so that you feel secure while walking. If your walking is badly impaired by knee or hip arthritis, a walking stick or walking frame may be very helpful.