Set the width of your outermost element to 100%.
It's down to how you construct your page as to where to apply that rule. You don't need to use <div> tags per se.
<div> tags are used to denote a logical division in content, you should not, as many people seem to do, scatter them about willy-nilly. There's no problem in using them, but like all HTML tags they should be used appropriately.
If, for instance, your site content is all within a table (erk!) then set the table width to 100%. If, on the other hand, your content is all within a <p> tag (boring page I guess) then set the width of the <p> tag to 100%.
By default, block level elements, such as <div> tags will take up the entire width available to them unless you specify otherwise.
With more complex layouts you mey need to use <div> tags to denote logical blocks of content. For example, a navigation block, a side bar, a content area. But you should always look to reducing the number of elements within elements. If your navigation is done with an unordered list (<ul>) then there is no point in putting that into a <div> element as there is nothing you can do to the <div> that you can't do directly to the <ul>.
I think I may have repeated myself a bit there and I'm not certain that what I have said answers your question directly. Perhaps what I am trying to say is that you should have a clear understanding of precisely
why one would use certain tags and thus understand how to create a
logical structure to your document.
<honk>*:
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