God told Abraham to leave Ur and go to a new land.
Abraham was told to leave his homeland and go to a place god chose for him.
Sacrifice him
because he was told by god
God told him to "go to the land that I (God) shall tell you," and Abraham journeyed to Canaan (Present-day Israel) and lived there (Genesis ch.12).
Because God spoke to him and offered him the eternal covenant. This was in the nature of a contract between Abraham and his descendants, and God. If Abraham agreed to trust God, and leave his home, and go searching for the 'promised land', and if he agreed also to remain faithful to the one true God of Israel, then in return God would bless all of his descendants and make them the founders of 'great nations'. Abraham was a Hebrew, and the founder of Judaism.
what were the messages that god sent to the prophet Abraham to tell his people?
God instructed Abraham to leave his family and homeland in Genesis 12.
to the Promised Land, Canaan.
Abraham eas ready to leave everything and go to the unknown land god was willing to lead him to.
I suppose you can consider it conditional in that Abraham had to leave his country in order for God to fulfill his promises. But as for all the blessings God promised (that were to be given once Abraham arrived in his new country), those were all unconditional.
God promised Abraham that if he would leave his homeland and listen to god, and only go where god would lead him, his desendents would be as the number of stars in the sky and the number of grains in the sand which are unaccountable.