When Rosh Chodesh Nissan coincides with Shabbos, in addition to the regular weekly Parsha, an additional two portion are read, Uvyom HaShabbos (Bamidbar 28:9-15) is read in honor of Rosh Chodesh and the special reading for Shabbat Hachodesh (Shemos 12:1-20).
Since there are two extra readings in addition to the weekly Parsha. The sixth Aliya is read until the end of the Parsha. The Rosh Chodesh reading is designated for the seventh Aliya, followed by half Kaddish and the HaChodesh reading for Maftir.
Additionally a special Haftorah is read from Yechezkel Chapter 45:18 – 46:15; Yeshayahu 66:1; Yeshayahu 66:23-24; Yeshayahu 66:23 (Chabad custom).
Half Hallel is recited at the conclusion of Shacharis and a special insert, Ata Yotzarto, is recited during Musaf.
At the end of the Davening, we begin to recite the Daily Nasi.