Hello and welcome to Android Forums!
Because the Galaxy S2 classification covers different models from different carriers, the forums have been split so not to cause any confusion. Select the forum appropriate to your phone.
Samsung Galaxy S2 (AT&T) - Android Forums
Samsung Galaxy S2 Skyrocket - Android Forums
Samsung Galaxy S2 (T-Mobile) - Android Forums
Samsung Galaxy S2 (Sprint) - Android Forums
Samsung Galaxy S2 (International) - Android Forums
On your phone, go to menu>settings>wireless & networks>usb utilities and select connect storage to PC. You should be prompted at that point to plug in your phone via USB (or unplug it, if it's already plugged in and then repeat the process.) Once it's connected you should see prompts to mount as storage, which once you accept, your phone's memory will show up on your PC as a removable drive. Open that location and create a folder called 'music' (without the quotes).
Now, find your iTunes library. Windows 7 puts it in C:\Users\<user profile>\Music\iTunes\iTunes Music. If there is enough room on your phone, simply copy all the contents of your iTunes library to the phone. Or, if you only want certain artists or albums, copy them one at a time. Since you have only 10 GB of storage internally on the phone, you'll need to edit your selections or get a 32 GB SD card.
If you have play lists in iTunes that you want as well, simply open iTunes and export the play lists as .m3u files and copy them to a folder on your phone called 'playlists'. Create it if it's not there.
When you turn off USB storage on the phone, Android will scan your new files and folders and your music will be there as well as your play lists.
