How do I Tell my Mum?!?

At the moment I live in Australia, New South Wales and when I finish School I am planning to move up to the US to go to UNI... I have a specific one in mind (CU - University of Colorado, Boulder) But im kind of a Mummy's little girl.. I have a mum, step mum, a dad and a step dad (due to divorces) and my Step mum is American (born and raised there) so i can easily get my residency :)) But the thing is... I don't know how to tell my mum.. I KNOW this is what i want to do... (Even though its not for a few years till its time to do it) But then all the doubts cave my mind in and I KNOW my mum will point out all the negativaties.. SO im not planning to tell her till im older and have DEFIANTLY decided ... But i need support .. how do i tell her... ?!?!?