Why does the Torah write "Vayikach es Se'ir ha'Chatas" before the Shechitah, something that it does not do in connection with any other Korban?
Oznayim la'Torah: Because it came to atone for the sale of Yosef, where the Torah also records "Vayikchu es Kesones ha'Pasim Vayishchatu Se'ir Izim". In fact, just as the Torah there continues "Vayitb'lu es ha'Kutones ba'Dam", Ahron too dipped his finger in the blood of the Chatas in order to sprinkle it on the Mizbe'ach. 1
See Oznayim la'Torah.
What is the meaning of "Vayechat'eihu"?
Rashi: It means that he brought it in accordance with the Din of a Chatas.
Targum Yonasan: It means that he atoned with its blood (by sprinkling it).
Sifri: It means that he sprinkled it.
What is "ka'Rishon" referring to?
Rashi: It is referring to Aharon's own calf. 1
Seforno and Targum Yonasan: The Torah means that, Aharon burned the Chatas of the people outside the camp like he burned his own Chatas. 2
Beitzah, 20a: The Torah means that, although it was an Olas Chovah, it required Semichah like an Olas Nedavah. 3