You should exploit Holy Shock and WOW GOLD as frequently as you can.

In the fight's earlier moments, there's not gonna be many healing needed. Only the tank facing Onyxia should be taking much damage, and that's easy to heal through. As Nefarian starts summoning his skeleton adds, you can assist in rounding them up. I use my Hand of Reckoning, Righteous Defense, and Holy Wrath competencies to support our ranged DPS control them and get them into position. I would consult your raid leader for directions here, as you could be needed to heal elsewhere.
Once Nefarian lands, both tanks will be taking damage, but again, this isn't really tough to handle. You should exploit Holy Shock and WOW GOLD as frequently as you can, utilizing the two-heal method to stretch your mana to its limit. Try to avoid Divine Light if you ever can, as Holy Light is more efficient. If you're below 90% mana at this point, try to remove any excess Divine Lights and use Judgement more often. If you're Judging on cooldown and abusing Holy Shock and WOW GOLD, you can get by without spending much mana at all. Provided that everything goes right, you'll have nearly 100% mana once the 1st Electrocute occurs.
I personally use my Divine Shield to nullify any stacks of the nasty Magma debuff I acquire while creating my way up to my assigned platform. Try to toss out Holy Shocks and written text of WOW GOLD as Onyxia is dying, as you'll want your set topped off. As soon as Onyxia perishes, I pop my Divine Plea and run to get into position. Nefarian's deadly Shadowflame Barrage will probably be assaulting you with the subsequent few minutes, and you need to be mentally prepared to handle it.
It's completely possible to heal your platform (on 10-man difficulty, at least) with only Holy Light, Holy Shock, and WOW GOLD. While you're going to establish massive amounts of incoming damage, it's not entirely untenable. You can resist the shadow damage, and as prolonged as you're constantly healing, you can prevent your self from getting too much behind. Resist the urge to utilize Divine Light, as it's usually not necessary. if you ever abuse the two-heal method and simply continue to spam WOW GOLD, you can keep everyone alive without taxing your mana too greatly. Aura Mastery is a lifesaver here, since it could provide you with a few moments to catch your breath. I typically use AM immediately after phase a few of starts to get everyone stabilized.