What instructions did God give to Moses to protect their first born?

Answer 1

If this is in regard to the last plague God would visit upon the Pharoh and Egypt, to kill Egypt's firstborn, then let us visit Exodus 12. God tells Moses to protect their firstborn to slaughter a lamb, and using hyssop to paint the top plate and sides of their doors with the blood so that the Angel of Death, in legend that was the angel Samael, will "Passover" the Hebrew people's homes without consequence. Thus, the Hebrew's first born were spared.

Answer 2

In Egypt, prior to the tenth plague, God told Moses to tell the Jews to sacrifice a lamb and smear its blood on the doorposts of their houses. Then, when the Angel of Death came that night to kill the Egyptian firstborn, it would know not to kill the Hebrew's firstborn too.