How to activate/unlock the sim of iphone 3gs?

It depends where you got your iPhone 3GS, if you got it through a authorized dealer, they will be able to fix that problem for you easily. If you got it second handed or from any third party companies, then you have a problem. The most common problem with iPhones are the "unlock" request, Apple do so to keep their phones lock for a reason, but there are lots of programs out there or even business services that will help you unlock/active your phone. Check google for iPhone unlock or activate iPhone, trust me.. its super easy.