What kind of permissions to change in wordpress

I am completely new to wordpress. I just downloaded it and wanted to install a theme and then I received following error:

Could not create directory.

Well I think this is a permission error. I could now enable and allow everything but I am scared about the security then. What permission are usually used with wordpress? I run my wp instance on a ubuntu 14.04, the site runs on an apache2 and the wordpress sub/directories are all root:root.