i've tried google and i've gotten nothing comprehensive

anyway i'm coding a div layout, and am using actual numbers "width: 490px;" etc for the positions. i've changed my screen resolution to 1024x768 because that's apparently the most common resolution, so hopefully it'll look good in most browsers. unfortunately it'll look rubbish in other resolutions so how do i fix that? been seeing percentages instead of pixels, to scale the site to fit all resolutions but how do i actually do that

here are the ones i'm trying to fix by the way:

<div id="left" style="position: absolute; top: 185px; left:700px; width:100%; height: 700px;>

<div id="header" style="position: absolute; top: 185px; left:95px; width:580px; height: 700px;">

and as mentioned, this is best in a 1024x768 screen

thank you