Cant upload Photos

Hi

I hope someone can help. Im getting the following error message when trying to upload photos:

Unable to create directory /var/www/vhosts/sites.county-homesearch.co.uk/httpdocs/wp-content/blogs.dir/15/files. Is its parent directory writable by the server?

Any ideas? Ive done a search on this forum and most people are saying change the file permission? How do i do that and what do i change it to?

Thanks in advance.
R