how do i activate an old iPhone

You'll need to contact your cell carrier to get a sim card that can fit in your 3GS assuming your carrier is a GSM carrier. Put it in your 3GS and turn it on, then follow the setup prompts if it was wiped, or just start using it if it wasn't.

You may be able to get a Micro Sim to Mini Sim adapter you can use to insert your 4S sim card into your 3GS.

http://www.amazon.com/Microsim-Adapter-Iphone4g-Convert-Regular/dp/B003XCCYV0

If its CDMA they would need to reactivate the phone directly to transfer your service to it. (no sim cards in CDMA phones), and then you can load your backup.

You can't restore a backup made with a newer version of iOS to a device running an older version. Since the 3G can't run anything higher than 4.2.1, your existing backup isn't going to do you any good.

As far as activating the old phone, go to your carrier and get a SIM card. Put it in the phone. Turn on the phone and plug it into your computer running iTunes. Follow the instructions.

Phil0124 wrote:

If its CDMA they would need to reactivate the phone directly to transfer your service to it. (no sim cards in CDMA phones), and then you can load your backup.

FYI: There is no such thing as a CDMA iPhone 3G or 3Gs.

KiltedTim wrote:

Phil0124 wrote:

If its CDMA they would need to reactivate the phone directly to transfer your service to it. (no sim cards in CDMA phones), and then you can load your backup.

FYI: There is no such thing as a CDMA iPhone 3G or 3Gs.

True.

As for the backup, yes, an older version of iOS cannot be restored form a new version backup.

You'll need to activate it through your carrier. Sync the old phone with iTunes so you have a backup, then restore the old phone using that backup to transfer your data.