Who am I that I should go to Pharaoh and lead the children of Israel out of Egypt? The time for Moses to assume the responsibility for which God had been preparing him had finally arrived. Without him being aware of it, God had chosen him for this special purpose, and all the events/happenings in his life that might have come across as mere coincidences had been divinely orchestrated to prepare him for this particular moment. This is essentially what God is telling him in response to his reaction. 
Moses’ reaction is not unique to him. It is a reaction that is typical of those that God elects for special missions. We encounter the same reaction in the call of the prophets Isaiah (cf. Isaiah 6:50, Jeremiah (cf. Jeremiah 1:4-6), and Amos (cf. Amos 7:14). God responded to these prophets’ protest by reminding them that he is the owner of the mission and that he will make them worthy of that mission. 
Moses was within his rights to be scared of the mission to which God was sending him. It was no small task that God was asking of him. It involved going to face the revered, almost dreaded Pharaoh. And with his recent crime of murdering an Egyptian, how could he risk presenting himself to Pharaoh, let alone leading the Israelites out of Egypt? God understood Moses’ concern, and moved to assure him of his support: “I will be with you since it is me who will be fighting those battles” (cf. Exodus 14:14).
The call of Moses, as would be the call of the many prophets that would come after him, was a difficult one. It was a call to do what appeared almost impossible. It was a call that demanded trust from him. Being that it was the first time that Moses was encountering God, he perhaps felt that God’s request was too much for a first encounter. However, God assured him of being present to him and to help him succeed. All that Moses had to do was to put his trust in God. God was going to do the rest.