Hi Hadiseh,
I've been away a while and just catching up on my mail.
Firstly, I no longer have a need to run the Linux server, as I am now involved with other projects, but for what it's worth I'll tell you how I got up to speed.
Firstly I got a couple of good books from the library and skimmed thru them and tried out the commands and got an overall view of how it all worked. For anything specific not in the books I googled and found Linux resource sites.
This is a good tutorial site.
http://www.ctssn.com/
As I was more comfortable with Windows, I downloaded and installed xammp on my Windows machine
http://www.apachefriends.org/en/xampp.html
I used dreamweaver with xammp to build my PHP/MySQL software pages and debug them on my home (Windows XP) machine. That way you only have one thing to learn at a time.
I also installed phpMyAdmin on my Linux box and found to simple to just upload and
test out my nearly finished sites.
http://www.php.net/ is a great reference site for PHP.
For anything I was stuck with I posted questions on forums, (mostly this one) and got great help. If you try first and get stuck, people and pretty willing to help you in my experience.
Some of the things I had to do, like setting up email, and DNS were quite tricky, but with a lot of persiverance, and forum help, I was able to do what I needed.
I didn't find it easy, but the answers are out there.
good luck!