I think that setting all to 777 is not the most secure option.
Basically, my favourite is to change the wordpress to the webserver's owner. Being:
sudo chown -R www-data:www-data /var/www/
So that the webserver is responsible for handling changes.
Also, you can change the rights to 755 for the whole /var/www branch
sudo chmod -R -c 755 /var/www/
Finally, allow your wp-content/ folder in 775 ( so that you can edit files from the WordPress backend without having to upload files via FTP. ( Not necessarily a best practice but useful from time to time ) via
sudo chmod -R -c 775 /var/www/WORDPRESS/wp-contents/
And you will have a reasonable setup with uploads, editing from backend and a minimum level of security.