Hi all,
I saw that it is possible to measure votage with arduino analog pins.I understood most of the hardware but I still dont understand how to calculate the appropriate resistors.here is the article i saw http://www.clarenceho.net:8123/blog/articles/2009/05/17/arduino-test-voltmeter.
I want to thank you in advance for your time. 
See Voltage divider - Wikipedia
It's a pretty simple concept and definitely worth your time to learn it fully.