Safety with PHP.

Hey you guys! I'm a new girl at this, and wondering on a question that might be simple for you. Yesturday I started to learn PHP and proud I showed it to a friend of mine, what I had learned. The first thing he did was check the safity of it and very easy he found the list over all the users of the server, f.ex. I'm glad I didn't upload the new pages as frontpages yet =) What can I do to prevent this and how can I build the pages safer than they are today? Perhaps it would be a good ide to learn basic Linux, I'm also considering to buy another computer to use with Linux. Greetings, Marthe -http://www.marthe.com (under construction!)