Build a website for different screen resolutions?

I built my website on my Mac in iWeb as I didn't have Dreamweaver, and iWeb is very easy to use. I got to many times where I wanted to do something that iWeb couldn't (like Spry menus), and elected to dump iWeb and move to Dreamweaver (which I broke down and purchased). I rebuilt the entire site the way I wanted, and I greatly approved. What has come to my attention is that I fixed everything to display on my 1440 x 900 screen. When I viewed the site on my 320 x 480 iPhone, my 1366 x 768 alternate computer, or my 1920 x 1080 TV, it looks horrible. When I designed the site in iWeb, the screen resolution didn't matter, it looked nice on everything. Every image, textbox, and table has been constrained with the 'style="position:absolute; top: Xpx; left: Ypx"' attributes/parameters. I noticed the only difference in my iWeb content is the use of

tags. I didn't use quite as many as iWeb did. Before I got rewrite the entire site again with addition

tags, I'd like to know that this is what I'm looking for. What do I need to implement to be able to view the site in any resolution without content distortion? I don't much care for the iWeb version of the site anymore, but it's the only thing that looks decent on 80% of the world's displays.