I have heard somebody mention a sacrifice that was made before the Mahabharata battle began. Please share any information available.
On the first day of the Kurukshetra war, there were countless unnamed warriors who die, before any named characters were killed. The Bhishma Parva describes what was generally going on:
And in that awful combat destructive of heroes, the sire slew the son, the son slew the sire, the sister's son slew the maternal uncle, the maternal uncle slew the sister's son, friend slew friend, and relatives slew kinsmen. Even thus the slaughter took place in that encounter of the Kurus with the Pandavas. And in that frightful and terrible battle in which no consideration was shown (by anybody for anybody).
But the first named character killed in the war was Virata's son Uttara, who was killed by Madri's brother Shalya (who was unfortunately forced to fight on the Kaurava side, as I discuss in this answer):
And Uttara also, the son of Virata, on a tusker with upraised trunk, rushed against the ruler of the Madras. Salya, however, succeeded in checking the unparalleled impetuosity of that prince of elephants rushing quickly towards his car. That prince of elephants, in great wrath, placing his leg upon the yoke of (Salya's) car, killed his four large steeds of excellent speed. The ruler of the Madras then, staying on that car whose steeds had been slain, hurled a dart, all made of iron, and resembling a snake, for slaying Uttara outright. The latter's coat of mail being cut through by that dart, he became totally deprived of his senses and fell down from his elephant's neck, with the hook and the lance loosened from his grasp. And Salya then, taking up his sword and jumping down from his excellent car, and putting forth his prowess, cut off the large trunk of that prince of elephants. His coat of mail pierced all over with a shower of arrows, and his trunk cut off, that elephant uttered a loud shriek and fell down and expired.
The other sons of Virata, the king who hosted the Pandavas during their year in hiding, were also killed in the war, along Virata with himself.
Uttara was the first one to die in the war but before even war starts one great legend who can end the war with only 3 arrows sacrificed himself for the good. Him name is Barbarika, he was the son of mourvi. Sri Krishna plotted the chart of all people, who can alone end the war in number of days with their skills. In them one is Barbarika. He sent the soldiers for Barbarika. Barbarika can and he explained how he can end the war with only 3 arrows. He got a boon from Lord shiva. If he put the arrows in the attacking position and start saying one mantra then the first arrow remembers everyone in the war, then second arrow remembers the friends or people whom we don't want to kill and the 3rd arrow will kill all the people except friends that 2nd arrow remembered. Krishna wanted to see his skills and he asked to cut all the leaves of a tree into two halves then he pointed 3 arrows and said the mantra. Arrow cut all the leaves and it is rotating around the foot of Krishna. Lord Krishna plucked one leaf before Barbarika pointing the arrows and kept it under his foot. Krishna you must be hiding one leaf there please remove your leg or it will go through your foot. He pulled out his leg and it cut the leaf. Krishna thought that if he became a part of the war then no one including Krishna will survive the arrows. Krishna asked Barbarika to kill one warrior before the kurukshetra war then Barbarika promised Krishna. Krishna went in and brought one mirror and he placed it in front of Barbarika. Barbarika understands and asked Krishna for one thing I came here to become a part of this war but I want to watch the war will you do that for me? Then Krishna replied saying even though you cut off your head you won't die untile the war ends. Your head and it's parts still remain alive and you can do things they do. We will place your head in a place where the whole war can be seen. Then Barbarika killed himself by cutting off his head. In this way one warrior died before it even starts.