How do you find a serial number to a gun that is registerd to you?

Look on the weapon.

If this is an instance of a stolen firearm, and you need the serial number for reporting, there are a couple different ways you might be able to find it. If you have a case the firearm came with (particularly with handguns), the number could be on a tag on the case. If you remember when and where you bought it, the shop you bought it from should be able to pull up those records.