Moosaa, the one to whom Allaah spoke directly, the strong and trustworthy.
We see that his call was directed to tawheed and carried the lights of guidance and wisdom. Moosaa was brought up and was raised in the palaces of the greatest tyrant who claimed divinity. He knew the various types of corruption, unbelief, tyranny, injustice and despotism from what he witnessed in the palaces. He saw things which it is difficult to imagine or think possible. He also saw the enslavement of his people, the Children of Israa'eel, and their humiliation, and their women taken as slaves, and the killing of their children, to such an extent that it surpassed any oppression known to mankind. Allaah, the Most High, says,
"Pharaoh exalted himself haughtily in the land, and divided its people into castes. He enslaved a group of them (i.e., the Children of Israa'eel), killing their sons and letting their women live. He was indeed one of the corrupt evil-doers." 
The people of Pharaoh were also people of shirk and idol-worship without a doubt.
So how did Moosaa begin? Was his call directed to correction of the 'aqeedah of this idolatrous nation? Or did it begin with a demand for the restoration of the rights of the Children of Israa'eel, and a struggle to gain rulership and authority from the hands of the tyrant, and at the head of them Pharaoh, who claimed divinity for himself?
The call of Moosaa was just like the call of his forefathers and brothers from the earlier Prophets. His Lord instructed him and inspired him with the principle of tawheed, and chose him to carry his revelation, and to establish worship of Him. Allaah, the Most High, says,
"Has not the story of Moosaa come to you? When he saw a fire and said to his family, 'Remain where you are for I have seen a fire. Perhaps I can bring a fire-brand from it or find someone who can show us the way.' So when Moosaa came to the fire his Lord called him, 'O Moosaa, I am your Lord. Remove your shoes, indeed you are in the purified valley of Tuwaa. I have chosen you as My Messenger, so listen to and heed what I reveal to you. Indeed I am Allaah. None has the right to be worshipped but Me. So worship Me and do not worship anything else besides Me, and establish Prayer for My remembrance. Indeed the Final Hour is certainly coming, which I keep hidden from all except Myself, in order to reward each soul according to its good or bad deeds.'" 
So this was the beginning of his Messengership; he was inspired and ordered with the 'aqeedah of tawheed. He was ordered to establish it in himself and to represent it in his life. Then he was entrusted with the duty of calling to this sublime principle and was sent by Allaah to the Pharaoh. Allaah furthermore explained to him the correct manner of giving da'wah and the wise way in which he was to face the Pharaoh. Allaah, the Most High, said:
"Go to Pharaoh who has transgressed all bounds in his haughtiness, pride, unbelief, and say, 'Will you not purify yourself from the sin of unbelief and be obedient to your Lord? And that I guide you to the worship of your Lord, so that you may submit fearfully to Him and obey Him.'" 
He also supported him with his brother Haaroon in order for the proof to be established to the utmost, and He taught them to use gentleness and mildness in da'wah since that is the best way to attain the guidance of those whom Allaah wishes to guide:
"Go both of you to Pharaoh, indeed he has transgressed beyond all bounds, and speak mildly with him that he might accept admonition and fear and be obedient to his Lord." 
So they carried out the order of their Lord and called him to Allaah, hoping for him to be guided and purified, so that he should be one of those who fear Allaah and fear and beware of the evil consequences of shirk and oppression. But he did not respond to this wise and composed call. So Moosaa manifested the great signs and clear proofs of his Prophethood and the truthfulness of his Messengership. But the despotic tyrant Pharaoh merely increased in his transgression and rejection of the truth:
"But Pharaoh denied the signs which Moosaa came with and disobeyed his order for him to fear and obey his Lord. Then he turned away from what he ordered him (i.e., obedience to his Lord) and instead worked evil and corruption, and he gathered his people and his followers and said, 'I am your lord, the most high.' So Allaah seized him with punishment for his latter and his earlier saying." 
 Soorah al-Qasas (28):4.
 Soorah Taa Haa (20):9-15.
 Soorah an-Naazi'aat (79):17-19.
 Soorah Taa Haa (20):43-44.
 Soorah an-Naazi'aat (79):21-25.