Just to clarify, You are trying to make a sidebar inside of a container that sits to the right? What you could do is...
CODE
<div id="container">
<div id="content"></div>
<div id="sidebar"></div>
</div> /* This closes container*/
Notice how the content and sidebar are sitting inside of a container. The container has not yet been assigned a set width, we will be doing this with the following code. If the contents that are inside of the container div (div content and div sidebar) were to have widths that are to wide for the container to handle; they will drop down and forced to display under the other and not side by side.
When saying left is so many pixels it should be working for you. ie with the above code
CODE
#container {
margin: 0 auto;
width: 700px;
}
#content {
margin: 0 200px 0 0;
}
#sidebar {
width:200px;
float: right
}
This CSS code will leave you with 500px of room to work with. This is because you take the container value which is 700 subtract 200 and then you are left with 500 pixels. The sidebar floats to the right and has a declared width so the browser knows what to do with the rest of the page.