As small as this baby is, you have your work cut out for you! Opossums are marsupials that give birth to several babies that then make their way to the pouch. There, they attach to a teat and continue to grow inside the pouch. It's likely that this little one won't survive, but you could try tube feeding it warm goat milk. Keep it warm and feed small amounts frequently. Look up opossums online, and get some info on babies (about the same age) with mom - then improvise. Good luck!