Help setting up the scales in AutoCAD ?

I am new to auto cad and am having trouble with setting the scales on it. I have set up a new page in A3, and i have dimensions for a model. I want this model in a scale of 1:5. Normally when i draw a line, it draws it in a 1:1 scale, then i change it to 1:5 and it still draws it in 1:1. I know it is because when i am dimensioning it, it still says 100mm, for a 100 unit line that i draw. any help?