Select anywhere on the grid to become the new 0,0 cartesian point

Hi

I really apologise if this is the stupidist already answered question ever, but i'm two nights into watching tutorials and playing with DS and have created exactly what I need (in 3d even so completely impressed by this software) but for the life of me how do you select any point on the grid in top view and say this is the new 0,0 from a cartesian perspective.

I have been able to start circles and rectangles etc by selecting cartesian coordinates from a point etc but if I start my objects in a random spot  how do i then select a point anywhere (in top view sketch) and say make this the new 0,0,0 for the whole drawing?

I have tried moving the grid with right click which seems like the right ball park but am not finishing it correctly? 

I know its a real newb question but any help would really be appreciated.

Thanks

Mark