How can I improve my eyesight natually?

None really. Vision therapy and some eye exercises can help certain specific conditions (see optometrist). Some vitamins can also help certain visual problems, but not recommended for routine prophylaxis.
Very little. For the most part there is no active way to improve the vision naturally. Of course eating a healthy diet, exercising and abstaining from tobacco, alcohol, and drugs are the most reliable but not very specific way of improving the vision.