"Then Moses answered and said, 'But suppose they will not believe me or listen to my voice; suppose they say, "The LORD has not appeared to you."' So the LORD said to him, 'What is that in your hand?' He said, 'A rod.' And He said, 'Cast it on the ground.' So he cast it on the ground, and it became a serpent; and Moses fled from it. Then the LORD said to Moses, 'Reach out your hand and take it by the tail (and he reached out his hand and caught it, and it became a rod in his hand), that they may believe that the LORD God of their fathers, the God of Abraham, the God of Isaac, and the God of Jacob, has appeared to you.' Furthermore the LORD said to him, 'Now put your hand in your bosom.' And he put his hand in his bosom, and when he took it out, behold, his hand was leprous, like snow. And He said, 'Put your hand in your bosom again.' So he put his hand in his bosom again, and drew it out of his bosom, and behold, it was restored like his other flesh. 'Then it will be, if they do not believe you, nor heed the message of the first sign, that they may believe the message of the latter sign. And it shall be, if they do not believe even these two signs, or listen to your voice, that you shall take water from the river and pour it on the dry land. The water which you take from the river will become blood on the dry land'" (Exo. 4:1-9).
Moses is still reluctant to embrace the task God has given to him. He wants to know how he will prove to the Israelites that he has truly come from the LORD. Moses' concern here is not unreasonable, though even after God puts every concern to rest, Moses still doesn't want to go (cf. 4:13). Thus, it makes me think (even at this point in the narrative) that Moses is more interested in finding an excuse not to go than in genuinely gathering important information to effectively fulfill his task.
It has always been the case that miracles from God have been used to confirm the divine origin of a message (cf. Mark 16:20). In Moses' case, he will initially be able to transform his rod into a snake and make himself leprous (as well as the undoing of those two acts). Additionally, he will have the power to turn water into blood. And this is only the beginning of the mighty deeds God would work through him! If one is able to demonstrate God's power in some miraculous capacity, then it implies divine approval upon the message being delivered (since it is assumed that God would not allow one with a false message to exercise such power).
"Then Moses said to the LORD, 'O my Lord, I am not eloquent, neither before nor since You have spoken to Your servant; but I am slow of speech and slow of tongue.' So the LORD said to him, 'Who has made man's mouth? Or who makes the mute, the deaf, the seeing, or the blind? Have not I, the LORD? Now therefore, go, and I will be with your mouth and teach you what you shall say'" (4:10-12). Moses claims he does not have the speaking skills for the job. God affirms that He, as the great Creator of speech and the organs necessary for such, will assist Moses. God then explicitly commands Moses to "go". Moses still isn't moving.
"But he said, 'O my Lord, please send by the hand of whomever else You may send.' So the anger of the LORD was kindled against Moses, and He said: 'Is not Aaron the Levite your brother? I know that he can speak well. And look, he is also coming out to meet you. When he sees you, he will be glad in his heart. Now you shall speak to him and put the words in his mouth. And I will be with your mouth and with his mouth, and I will teach you what you shall do. So he shall be your spokesman to the people. And he himself shall be as a mouth for you, and you shall be to him as God. And you shall take this rod in your hand, with which you shall do the signs'" (4:13-17).God had put up long enough with Moses' reluctance. His anger is kindled and Moses stops arguing and wisely prepares to take action. God affirms that Aaron will assist Moses. Moses will be the authority over Aaron (like a God, so to speak). Although let us quickly point out that the instructions Moses will give to Aaron ultimately originate with God.