KWASI - I have been studying of late about the atonement and the law of sacrifice.I want to know why it was so necessary that such sacrifices required the firstling or the only beggotten in the case of the saviour, secondly why blood? Couldnt anything be used to substitute it?

JOEL - In order for mankind to remember constantly the great atoning sacrifice that was to be made of Jesus, God gave to Adam and his posterity the law of sacrifice, which was observed until the coming of the Son of Man in the flesh.
An explanation of the true meaning of sacrifice came to Adam one day when he was offering unto the Lord the firstlings of his flocks. He was informed by an angel:

"This thing is a similitude of the sacrifice of the Only Begotten of the Father, which is full of grace and truth. Wherefore, thou shalt do all that thou doest in the name of the Son, and thou shalt repent and call upon God in the name of the Son for evermore." (Moses 5: 7-8)

The firstling of the flock was used because it represented the firstling of God the Father, or Jesus Christ. From the very beginning the offering of the sacrificial animal was regarded by Adam and his successors as an act symbolical of the sacrifice of the Lamb of God. The shedding of the blood of the lamb represents the shedding of the blood of Jesus both in the Garden of Gethsemane and while on the cross as he atoned for our sins. Symbolically our souls are cleansed by His blood.

"Ye must be born again into the kingdom of heaven, of water, and of spirit, and be cleansed by the blood of mine Only Begotten, that ye might be sanctified from all sin . . . [for] by the blood ye are sanctified." (Moses 6:58-59)

Jesus, being God, and by virtue of His being the only begotten son of God in the flesh, made it possible for him as half mortal to shed blood and as half immortal to survive the terrrible punishment He suffered to atone for our sins.
Jesus sealed His testimony of the plan of God with His blood, which also caused that His followers would honor and remember Him for what He did; as we do when we partake the sacrament with the bread and water representing His body and blood.

And now for what I think is the real answer to your question. We really don't know why Jesus had to do this in the way he did; why the only begotten; why the firstling; or why blood had to be spilled instead of something else; because to ask this assumes that someone sometime decided that this is the way things had to be done to bring about an atonement. Laws like The law of sacrifice and the law of justice are eternal. They have always existed. It's like asking why does God exist? God the Father was in control of it all, but mostly it is an eternal law, that must be followed by God and all His children in all universes past, present, and future.