Drupla installing issue: "sites/default/files is not writable"?

Hi,

I am trying to install Drupal on Gentoo/Linux with nginx web server.

However installer found an error of "Requirements problem" - "The directory sites/default/files is not writable."

Please inform how to grant permissions to web server? I am quite lost regarding web server permissions...

If any additional information is needed please let me know.