So you have a large iTunes library and now you have a Samsung Fascinate -- but how can you make them work together to synchronize your music? The following guide shows you how to quickly and easy set up your computer to sync your Samsung Fascinate with music from iTunes.
The first section details how to acquire a small piece of software called "iTunes Agent", that allows you to use your Fascinate with iTunes on a Windows-based computer (we are assuming that you already have iTunes installed, and have an existing library of music). Mac users can achieve the same results by installing a program called iTuneMyWalkman.
http://ilari.scheinin.fidisk.fi/itunemywalkman/
Download and install iTunes Agent
http://ita.sourceforge.net/Once installed, the application runs in the system tray and monitors iTunes
Plug the Fascinate into your computer's USB port and mount it (figure 1 above)
Create a folder on the mounted Fascinate called Music
The second section, below, details how to create a configuration for the Fascinate in iTunes Agent's Preferences (figure 2 for all steps below).
Right-click on the iTunes Agent icon in the system tray
Click Preferences
Leave all defaults checked and click the New button
Give the device a name (Fascinate in this case)
Leave Synchronize pattern: set to iTunes
Click the Choose button and select the Music folder you created on the Fascinate
Click the Create button and select the Music folder you created on the Fascinate
Click Save and then Close
The final section explains how to add music to your Fascinate (figure 3 for all steps below).
Launch iTunes and you will see a playlist folder called My Devices, under which you will see the Fascinate
Simply drag the music you wish to synchronize from your iTunes library to your Fascinate into this playlist
Finally, right-click on the iTunes Agent icon in the system tray and select Synchronize devices
Music is now copied to your Samsung Fascinate and can be played using the Fascinate's Music application.
I did not Write this i got it from here
http://www.knowyourcell.com/samsung/samsunggalaxys/galaxy-s-guides/477037/how_to_sync_the_samsung_galaxy_s_with_itunes.html 
