Sites' positions in our search results are determined based on hundreds of factors designed to provide end-users with helpful, accurate search results. These factors are explained in more detail at http://www.google.com/competition/howgooglesearchworks.html.
In general, webmasters can improve the rank of their sites by creating high-quality sites that users will want to use and share. For more information about improving your site's visibility in the Google search results, we recommend visiting Webmaster Academy which outlines core concepts for maintaining a Google-friendly website.