Leviticus 9:7

But when Aharon saw at the corner of the altar the form of the calf, he was afraid to approach to its side. Mosheh, therefore, said to him, Take courage, and go near to the altar, fearing not, and offer thy sin offering, and make atonement for thyseIf and for the people, and perform the oblation of the people, and make atonement for them, as the Lord hath commanded.

secure atonement for yourself and for the people i.e., you are required to atone for yourself and for the congregation. You will secure atonement for yourself by the sin-offering of a bull; afterwards, you will bring the nation’s offering and secure atonement for them. No man can atone for another until he is cleansed of all sin himself.

DRAW NEAR UNTO THE ALTAR. 8. AND AARON DREW NEAR UNTO THE ALTAR AND SLEW THE CALF. It seems to me in accordance with the plain meaning of Scripture that Moses said to Aaron: “Draw near to the north side of the altar and offer there the sin-offering and the burnt-offering, for they are to be slaughtered on the north side of the altar.” Moses, however, said it briefly since Aaron already knew [that these offerings were to be slaughtered on the north side of the altar].
Our Rabbis in the Torath Kohanim were, however, prompted by [the use of] these expressions to draw a parable [and to say]: 8. The Rabbis were prompted by the use of the phrase “draw near,” which Ramban explained to mean “draw near to the north side of the altar.” “To what can this be compared? To a human king who married a woman, who in her shyness [did not dare to enter] before him. Her sister then approached her, saying, ‘My sister, why did you enter this matter? Was it not in order that you serve the king? Embolden yourself and go in to serve the king!’ Similarly did Moses say to Aaron: ‘My brother! Why were you chosen to be the High Priest? Was it not so that you minister before G-d? Embolden yourself and come and do your priestly activities.’ Some Rabbis say that Aaron saw the [horned] altar in the form of the bull [which — as stated in Psalms 106: 20 — Israel had worshipped], and he was frightened by it. Then Moses came near and said to him, ‘My brother, Aaron, do not be afraid of that which you fear. Embolden yourself and come near it.’ This is why he said, draw near unto the altar, and [Aaron] drew near unto the altar — with zeal.” The reason for this [apparition which Aaron saw in the altar] was that since Aaron was the holy one of the Eternal, having no sin on his soul except for the incident of the golden calf, therefore that sin was firmly fixed in his mind, something like that which is said, and my sin is ever before me., 51:5. It thus appeared to him as if the form of the calf was there [in the altar] preventing his [attaining] atonement [through the offerings he was to bring]. That is why Moses said to him, “Embolden yourself so that you should not be of such humble spirit,” for G-d has already accepted his works. Other scholars explain that it was Satan who showed him this apparition, just as they have said there in the Torath Kohanim: 3. “My brother Aaron, although G-d has agreed to grant atonement for your sin, you must nonetheless close the mouth of Satan [through your offerings], lest he accuse you when you enter the Sanctuary etc.” And the expression, and make atonement for thyself, and for the people means as follows: “draw near unto the altar to bring all the offerings, and offer first thy sin-offering, and thy burnt-offering, and make atonement for thyself first with thy offerings, and for the people afterwards, through bringing their offering and atoning for them by means thereof.” Thus Moses taught Aaron that the guiltless should come and effect atonement for those who are guilty.

קרב אל המזבח, “approach the altar;” why did Moses have to command Aaron to do this? Satan at the time had appeared to Aaron in the guise of a calf, and he had become afraid that he no longer was qualified to serve as priest, or certainly as High Priest. Moses reassured him that he had nothing to fear.

וכפר בעדך ובעד העם, “and make atonement for yourself as well as for the people.” This verse requires further study, as this offering was only on behalf of Aaron, not the people. We read further on in verse 15 and 16 of this chapter about the offering presented on behalf of the people. Seeing that this was so, why did the Torah in our verse include “the people?” We may answer that the Torah mentioned this in order to explain why Aaron first had to offer his personal sin offering, before being able to act as the agent of the people. As long as he had not obtained atonement for his part in the golden calf, he could not function as the people’s agent in this respect.