How do i prevent spider veins?

You can't. This is just superficial capillaries that have developed closely under the surface of the skin and that is just what your body does. When you are older, you may have a good job and be able to pay for some laser treatments for these. Sclerotherapy and laser therapy are the most commonly used treatments for spider veins. If these become larger and then become varicose veins, other treatments are needed such as the compression stockings, etc.