Noob question about being hacked

Hi all

I am managing a small website for a friend and I got hacked. I wonder if anyone can help me identify how and prevent it from happening again?

What happened - someone modified functions.php. They added some code to the file, then renamed it to functions.php123.

How can I see how they managed to get in to that file? What do I need to do to prevent them from getting in again?

Thanks