help understanding fixed layout

Hi , thanks for reading ,
I've got to grips with basic html and css ,
Now i'm moving on to layouts , i had it looking nice ,
but when i moved the viewer window it all moves around ,

so come to the understanding i want fixed layout
which seems to be the case in most the websites i like
does that mean i have to stick to fixed pixel values ???
is there no way of creating a percentage of the screens resolution ??
then the layout stays fixed in relation to view port ,
but automatically scales to the full screen size ??
make sense ??

if not are there any good guidelines for fixed layout
anyone know average users screen size ? type of stuff
size of an ipad screen ??

If you're wanting your page to shrink and expand in relation to the browser window then you need a fluid layout.

here (http://www.smashingmagazine.com/2009/06/02/fixed-vs-fluid-vs-elastic-layout-whats-the-right-one-for-you/)is a good article about fixed and fluid.