How to activate the used iphone 3gs?

Insert your SIM. You can not activate an iPhone without a SIM. Turn it on. If it is running iOS 5 or higher and was returned to its default settings before you got it, then it should prompt you to go through the activation process.

If it is running a lower version of iOS, plug it into your computer running iTunes and activate it with iTunes.

If it continually tells you that the activation server is busy, try again later, that most likely means the phone was hacked to unlock it. You will need to contact the carrier the phone is locked to and see if you qualify to have it unlocked.

If it tells you to insert a valid SIM from a supported carrier, that means the phone is locked to a carrier other than the one you are trying to use. You will need to contact the carrier the phone is locked to and see if you qualify to have it unlocked.

Unfortunately this sounds like a phone that was hacked, or jailbroken. We cannot help with jailborken phone. You're best to return the phone and get your money back and buy one from a legitimate source.