HTML woes

I was going to blog about my trip to the Philadelphia International Flower Show, but I got sidetracked by trying to change my background picture using a url instead of uploading the pic to blogger.  I would like it to be centered, fixed and take up the whole screen no matter what size your computer screen is.  If you know what I'm talking about and you know how to do this, LET ME KNOW!!  And in general, if you're an html/CSS wiz, I am terrible at it and totally cheat/fake/lie my way towards a design.  Please teach me the way.