In your quest log (press L), only do the stuff under "primary quests". If you only want to do HoM, everything else is a huge waste of time.
Looking back, I wasted an eternity in Nightfall by playing in the typical Wow style of grab all the quests in the area, do them and come back.
You can also use the wiki from the start to tell you exactly what to do. Nightfall starts here.
Just go through it with the "Preceded by" and "Followed by" on the top right.