I am a citizen and need to know how can I make a work contract for a nanny that is over seas?

You could potentially use the H-2B process to hire a nanny but quite honestly it will be difficult. You would have to demonstrate there is no worker in the United States qualified for the position, which will be difficult to do. The amount of time she would be authorized to be here is also of short duration as it H-2B is essentially for seasonal or project specific temporary employees. You can read more about H-2B at http://myattorneyusa.com/work-visas.