I will help you.
What's your current network provider?
I guess your iPhone 3GS is locked to your previous network provider.so you will need to unlock your mobile phone.You can easily unlock your mobile phone like using unlock code,software etc..If you avail Remote unlocking service,you need not to jailbreak your mobile phone.If you are interested in this method visit http://www.classicunlocking.com/apple-iphone-3gs/rs17wp9/
Once unlock your iPhone ,you will be able to use anywhere in the world. I hope it will be helpful for you.