i just got a new mustang how do i paint it a new colour

 

Take it to a custom paint shop and let them do it.

Not to be a ***** but since you don't know how to do the job tells me that you don't know what's involved.
To do a COMPLETE color change is a very complex job.It entails removing everything that comes in contact with or covers the paint.All glass,weatherstrip,mirrors,doors,both bumpers,inner fenders,door trim,trunk liners,hood,engine,radiator,etc.You get the idea.
The reason this has to be done is to get good coverage with the paint and good preperation of the surface.Skip either one and the paint will come off sooner or later.
If this is going to be a project car you can remove all the stuff that's needed to be removed at home and have the car moved to the paint shop,,,just like those damned custom car/bike shows do. One hour of work,some cussin' an' screamin',and they have a show car.

That is to do a "like new" repaint but unless the new color is completely different a good shop can match the color and you can get by with just painting the visible areas.

 

If it's new, why didn't you get it in the color you wanted?