How do I redeem the mileages of United Mileage Plus for an Air Canada ticket?

I have an amount of mileages in my United Mileage Plus. Now I want to redeem the mileages from this card for an Air Canada flight ticket. Where should I go to redeem these mileages? Is there a website to do so? I have tried to check out AirCanada.com but they do not have the option for redeeming United Mileage Plus. Please help. Thanks a lot.