websites "hacked" - how do I make changes to prevent this happening again?

[Nett Sales]

Hi,

We've got about 7 websites all created with various Genesis themes including our own www.nett-sales.com site.

Over the weekend someone replaced the head.php file in the child theme directory of two of our sites to display a "hacked by" image and text. I was able to go in and replace the head.php with a copy from our backup and got the sites back very quickly.

But the question is how do I prevent this from happening again?

I guess it's something to do with permissions, but I'm not sure where to start... Is there a good resource I can read on setting the appropriate permissions on files / directories that anyone can point me towards? Preferably at Newbie level please