I spread my views wherever I can. If someday threecoves here will get a job to make a site with ads he will hopefully remember what I said about fixed ads and help make the internet slightly better. But enough of that.
I take it you want some kind of ad placed to the top left, well what you would use is the float element. You say you are a beginner so I will do it very basic.
I could do some precise code and say "paste this and it will work out", but like the proverb goes... Give a man a fish and you feed him for a day; teach a man to fish and you feed him for a lifetime.
HTML:
<body>
...
<div id="wrap">
<div id="column1">
<img src="banner.jpg" alt="" border="0" />
</div>
<div id="column2">Main content</div>
<div id="column3">Sidebar</div>
</div>
...
</body>
CSS:
#wrap { overflow:hidden; }
#column1 { width:160px; float:left; }
#column2 { width:500px; float:left; }
#column3 { width:250px; float:left; }
VoilĂ .
As a sidenote you shouldn't make pages wider than 1000px since that will result in screens with 1024x768 have to scroll sideways, so there is a limit to how big banners you can have while still have room for content. Common sense may have to be applied.