I found a ipod 8gb it already had a unlock code in

Normally if you don't know the unlock code it's because you forgot it, and you can simply connect the iPod to the first original computer that it synced with through iTunes, and it will unlock.

However, since you don't know the code because you found it, the only way to get past the unlock screen is to restore the iPod. This deletes all of the data on the iPod. So, if you are seeking to return the iPod to it's owner, you may not be able to as the owner information/address (provided there was any) may be deleted.

To restore it iPod, do the following:
1) Connect the iPod to your computer running iTunes with a sync cable
2) Click "Restore"
3) The iPod will reboot once it is successfully restored, allow it to reboot

In iTunes, notice at the left the icon of the iPod. If there was a backup made of this iPod prior to the restore, you may be able to right click that icon and choose "Restore from backup".