What can you do to prevent getting a strong dizzy rush of blood to your head when you stand up and why would it start happening in the first place?

If you're getting dizzy when you stand up it's probably not a rush of blood to your head, but rather the opposite - a sudden drop of blood pressure. If you're a teen and growing rapidly then that's a common cause. If you're not a teen it might be worth talking to a doctor. Keeping fit can help with the problem.