The short answer is, you are going to have to gear for healing, as it requires completely different gear than feral. Though if you don't have a rogue in your group, I am sure you'd be able to pick up Feral gear while your gathering healing gear. Each PuG is different, just ask if you can roll on things for your offspec, but dont take gear from people who may need it for the instance you are doing. Most people are understanding in 5 mans that you will want gear for multiple specs.
Anyways, I've compiled a short list of links that I believe will give you a hand understanding the Druid as a healing class, and available options from lots of different players.
To start, here's a quick summary of what people view druid healers as:
Resto Druids: Druids are perhaps the
most necessary healer in any 10/25
composition. Mostly because of
Blizzard's encounter/item design
during and after Ulduar. After Burning
Crusade, Druids were trying to find a
niche because Blizzard nerfed their
insanely awesome Lifebloom. I think a
lot of Druids went into Wrath looking
to continue being tank healers, and
it's something they're still good at.
However, they found their niche with
raid healing through something called
Rejuvenation Blanketing. Basically, it
just means that they put Rejuvenation
on as many raid members as their
haste/positioning allows. Blizzard
reinforced this through incredibly
strong set bonuses for T8 and T9 that
boost the power of Rejuvenation
incredibly. In addition, the pulsing,
constant raid damage of a lot of
Ulduar/ToC fights basically plays
right into Druids' hands. Rejuv
blanketing puts out a stunning amount
of throughput that is almost
unmatchable. Like I mentioned, rolling
HoTs on tanks is also a strength of
Druids and really evens out tank
burst, though I seldom assign them to
tank healing unless very necessary.
They can tank heal just fine, but it's
a waste of their true power.
From: Druid vs The Healing World
There's also been an incredibly useful survey of Druid healers based on a survey called "Circle of Healers", with many blog post responses:
Druids
- Lathere, Druid, Restooration, Hots & Dots
- Kailanii, Druid, Restoration, Sheep This
- Kotakh, Druid, Restoration, From Miss Medicina's Blog Comments
- Sammich, Druid, Restoration, Kungaloosh
- Kaelynn, Druid, Restoration, Dreambound
- Aanthe, Druid, Restoration, The Angry Healers
- Verile, Druid, Restoration, Rejuvo
- Sersokhi, Druid, Restoration, Sushi-Cookie
- Kochi, Druid, Restoration, Rambling Dwarf!
- Tatia, Druid, Restoration, Dots & Hots
- Isla, Druid, Restoration, Unreal Realities
- Dysmorphia, Druid, Restoration, Dys-Morphia
- Hermia, Druid, Restoration, Tree Of Doom
- Ikkulu, Druid, Restoration (PvP), DorkArt
- Firewood, Druid, Restoration, Tree Haelz
- Bellwether, Druid, Restoration, 4 Haelz
- Lissanna, Druid, Restoration, Restokin
- Forrestump, Druid, Restoration, Forrest Stump
- Wingood, Druid, Restoration, Evian Tree Bear
- Elsen, Druid, Restoration, The View Through the Branches
- Killh, Druid, Restoration, WoW Frost Mage / Piou Piou Blog (French)
- Oestrus, Druid, Restoration, Divine Aegis
- Buel, Druid, Restoration, Buel's World
- Tazha, Druid, Restoration, Wild Growth
- Innoue, Druid, Restoration, Druid Tree
Other topics that may help:
Resto Druid - Starting raiding mana regen numbers?
What spellpower should a resto druid have before raiding
Building a Balance/Resto spec — DPS while healing in heroics?
You might be fine tanking normal 5-mans in your DPS gear (not Heroics) but if you want to get anywhere, you will need to pick up some more gear. Look for high stamina plate pieces with Dodge, Parry, or bonus armor, and shy away from crit.
Yes, you should absolutely change your gear when going from a DPS spec to a Tank spec (or vice versa, some feral druids excluded).
Best Answer
For leveling I suggest that you develop your own spec. Just take whatever you think will be fun. Max DPS is a non-issue - or at least should be. If you will start raiding with that toon, get a raiding spec and adjust it - just so that you can start developing a feel for it.