The funny thing is that when you bring Onyxia's head to Bolvar, Katrana Prestor is probably going to be standing right next to him, and she is Onyxia.

Gotta love WoW game mechanics.

To complete the entire quest up to this point, you need to do the following:

1. You begin in the Burning Steppes with the quest Dragonkin Menace, which is pretty easy.

2. "The True Master" chain comes right after step 1. You:
a. Talk to Salomon in Lakeshire
b. Talk to Highlord Bolvar Fordragon in Stormwind
c. Talk to Lady Katrana Prestor who is just across from Fordragon
d. Talk to Salomon in Lakeshire again
e. Talk to Marshal Maxwell back in the Burning Steppes
f. Talk to Ragged John at the Flame Crest
g. Talk to Marshal Maxwell again

From here you need to go to Blackrock Depths and talk to Marshal Windsor. He will tell you to return to Marshal Maxwell again with the quest Abandoned Hope.

At this point, it seems the quest just runs into a dead end because Marshal Maxwell does not give you any further quests. You need to go back to Blackrock Depths and kill mobs there for a random drop called A Crumpled Up Note. Right-clicking the item will set you back on a quest to Marshal Windsor.

Windsor gives you the quest A Shred of Hope, which is to retrieve lost information from Golem Lord Argelmach and General Angerforge. You return this to Marshal Windsor.

The next part is a bit hard - you need to escort Windsor from his jail cell. Make sure the area is cleared of mobs before you do this. The quest is called Jail break!

After escorting Windsor out, go talk to Marshal Maxwell. He will give you a quest called Stormwind Rendezvous, which can be done solo if you stay away from the battle.

Talk to Bolvar just after the fight. He will give you The Dragon's Eye. This means you have to go to Winterspring to find Haleh.

After this, go to Upper Blackrock Spire and kill Drakkisath for the Drakefire Amulet. Kill Drakk and loot the blood.

Return the blood to Haleh in Winterspring and she will give you the key to Onyxia. The Drakefire Amulet attunes you to Onyxia's lair, which allows you to kill Onyxia and return the head to Bolvar in Stormwind.