to unlock your device prior rooting is required!
a very simple solution provides the freeware tool Android Root Tool (scroll down to the bottom and dowload the tool, you can switch to english language after you started the tool!).
supported devices are:
NOTE! AV software alerts you because of the several exploids included in the tool. those exploids are nessessary to root your android device and are no harm to your computer whatsoever!
after you rooted your device you are ready to unlock it with the help of this free App:
Google Play - Galaxy S Unlock
works with Galaxy S, Galaxy S II and all variants, also Galaxy Tab and according to a user even with Galaxy Note.
unlock.it is your cell phone online unlocking service!
use your cellular phone with EVERY provider and make your mobile phone fully unlocked.
remove gsm sim lock - use our simple and free service!