I have a week eyesight of -2.. how can i improve it?

No simple fix: There are no eye exercises or vitamins to make you see better. You have 3 viable choices: glasses, contact lenses, or refractive surgery(lasik or prk). They all work well.

Use correction: You can start with glasses or contact lenses. More aggressive treatment may involve laser refractive surgery. There is no good evidence ortho-k can provide any significant improvement to the vision as the effect wears off once you stop treatment and there is an increased risk for infection.