This is a commonly asked question by new wordpress users. So I thought I will share some facts here with you.
The Style of your WordPress Website is decided by the Theme you have chosen. So the width depends on the current theme. The theme would have the width in its stylesheet.So if you want to change only the width, without changing the theme, you have to edit the stylesheet. But to edit the stylesheet, you should be good in CSS.
Choose the right theme
Another easier alternative is to choose the right Theme for your wordpress website. You can change your theme in WordPress->Appearance->Themes page.
Choose a theme that has the width you need. The “install themes” tab shows different options to select the type of the theme. You can even select Fixed width themes or Flexible width themes.
There are a number of themes that allows customizing all the aspects of the theme. That includes width, height, background color, header image logo etc. The customization page of such themes would have options to edit each of these aspects.
Thesis theme by DIYTheme is is one of such highly customizable theme. You can choose whether you need single column or double column layout. Moreover, you can choose the width of each column too.
So if you want to customize the width of your wordpress website, the easiest option would be to install a customizable theme