Being plate-wearers, STR is actually pretty important as a cleric if you don't want to be encumbered constantly.
If you've got a HE cleric, you'll want to start on this
line of quests (Tunare Symbol quests) for the free clicky root, which will save your butt and save a spell slot. (Make friends with a druid or wizard for the third quest unless you're cool with going back and forth across Norrath several times on foot.)
Armor-wise, doing the
Bracers of the Reverent quest twice might be in your best interests (and if you want to be a golden cleric, do the full set of armor quests!). Later on, you can go for the
Aegis of Life, which would require some faction-work and lots of travel/camping, but it's a back slot/secondary item with +10 wis so it might be worth it!
For weapons, you can try the
Bonethunder Staff quest for cool battle cleric bonus points, or attempt the quest for
Shining Star of Light to get a 1HB with a cool name. You will also want to do either
Paw of Opolla or
Testament of Vanear for your offhand if you go the 1hb route.
Or you can buy some cheap stuff in EC, but that's kinda boring...plus you need to save up for all your spells.
These quests I've named are all stuff you can complete by mid-thirties with help. The Bonethunder Staff can be obtained before level 20 if you choose to use the Regurgitonic rather than kill Mammoth.
As for playing tips...
* Starting at 24, use heal macros and load more than one heal spell. By this point, group members will have larger disparities in HP, so you use your big heals for melee and small heals for casters (usually).
* You do not have to just sit around and heal, you can also root-CC, stun, and fear. That last one is useful if you're playing with someone who has some form of snare, like a druid or ranger who can only fear animals. You can also nuke, but only the undead nukes are mana-efficient on a cleric. Cleric nukes are decent in the early levels.
* Learn aggro management as a healer. A lot of p99 players will discuss what tanks can do to hold aggro (good gear, procs, well-timed taunts, other melee types holding back or using evade properly), but your timing and choice in spells makes a pretty big difference in the first few seconds. Big heal = big aggro, but when you drop that big heal can either hurt or help your tank. In later levels, you will have to pay attention to when the monk puller is FDing, who is engaged with what and for how long, and be able to guesstimate how much aggro you've accumulated over the course of a fight.
* Always keep Divine Aura loaded in your spellbar, and maybe Divine Barrier if you have the space to spare. A wipe's a wipe, but if you're trying to buy time for the tank to get something off you, DA is irreplaceable. You should be one of the last ones to die in a group, followed only by a class with FD, because if the cleric dies it'll be a miracle if the rest survive unless you have secondary healers who know what they're doing.
* People are gonna ask you for rezzes in far off places when you're high enough. You're allowed to say no. Or yes. Or wander into zones and offer free rezzes for no reason. Or go anon and ignore the world.
* Hoard all the peridots.