I would like to create a Healer. Which class should I pick?

Healing means not just being able to restore hit points. A skilled healer is also able to remove Curse, blindness, paralysis and various ailments affecting party members during an encounter. A Healer is responsible for the well-being of a group in the broadest sense, maximizing the efficiency of the whole team and contributing to its success in an indirect manner. And of course, at higher level a Healer must also be able of bringing fallen comrades to life.

Healing is critical to the success of any adventure, but few players accept to be limited to the role of pure healers (also called "Nannybots"). Prevention is also part of the job, since, after all, a dead monster won't cause any wound. Crowd-control and damage-dealing are secondary purposes of a Healer, limiting the damage suffered by the party before it ever gets inflicted. Finally, healers are often able to cast buffs, various benefits improving the abilities of fellow party members. Strengthening allies is another way of avoiding damage.

Five classes are able to perform healing on themselves and fellow party members: Bards, Paladins, Rangers, Clerics and Favored Souls. Other characters are able to heal but only using disposable wands and potions, clickies or Dragonmarks, a solution suitable in an emergency but not for your daily dungeon exploration. Since every encounter is likely to inflict damage on characters, a healer is almost mandatory to any group. It's often better to have more than one, and healers are usually sought after when party members are gathered.

Partial Healers
Partial Healers are classes able to heal, but not able to do it as their primary purpose.

Bards
The incarnation of versatility, Bards are jack-of-all-trades naturally bringing magical means of healing. They get Cure Light Wounds spell as early as level 1 and many others later (up to Cure Critical Wounds at Bard level 10, and various Mass variants of lower level spells). They may also neutralize poison and remove curses. Yet, Bards lack spell points for any extensive healing and tend to use their spell points for buffs and crowd control spells instead (fitting more in damage prevention than cure). Even if some enchanted items may make up for their lack of spell points, thus increasing their healing abilities, Bards are never able to raise fallen party members. Bards make decent backup healers, not only because they have a large selection of spells to choose from, but also because their purpose is generally to support and assist the party in the first place. The Spellsinger bard Prestige Enhancement gets bonus spellpoints and a song which reduces the spellpoint cost of using all spells, which generally make them the best choice for a bard that focuses on healing.

Paladins
Paladins are another kind of healer, but certainly not as a primary role. Paladins get healing spells (up to Cure Serious Wounds) but lack spell points to cast them often enough (even more than Bards). Yet, they also benefit from Lay on hands ability several times between rests, a powerful, spell-like ability healing many hit points on any ally or the Paladin himself. Lay on hands is great because it's instantaneous; no interruption is possible through enemy attacks and loss of concentration, and does just require a line of sight to target. Lay on hands also works on Warforged characters, notoriously harder to heal. Paladins follower of the Sovereign Host may get the Unyielding Sovereignty enhancement at level 6, allowing a complete restoration of any character once every 10 minutes (irrelevant to Shrines!). Elven paladins may opt for the Undying Court and get a free Raise Dead / True Resurrection instead.

Paladins enhance a party thanks to their powerful Aura, but make fair healers at best. They are mostly found in the thick of battle where they are too busy to help fellows. Even if they can act as secondary healers from time to time, they are more often using their healing abilities on themselves, becoming self-sufficient fighters and leaving breathing space for the main healer of the party. Yet, through enhancements, it's possible to turn a Paladin into a healer, even to the point of letting him raise dead party members.

Rangers
Rangers are healers of the worst kind, yet number among the five classes able to perform this role without relying on disposable items. Rangers get three cure wounds spells (light, moderate and serious) but these are of spell level 2-4 respectively; a Ranger may only cast his first Cure Light Wound spell at level 8, and only five times per rest due to low spell point reserve. A Ranger gets as many spell points than a Paladin, but has no extra Lay on hands ability or anything similar boosting his healing talents. This is clearly not enough to perform even basic healing on the Ranger character, but of course it can't hurt to have another character healing a few extra hit points before a rest shrine.

True Healers
The most common healers are Clerics and Favored Souls.

Clerics
Cleric is the main healing class of DDO. Clerics get healing spells for free at each spell level (this is called Spontaneous casting) in addition to "regular" spells they can choose. It means a level 1 Cleric has the Cure Light Wounds spell and two other spells of his choice for a total of three. They still have to assign spell slots if they want to cure other ailments like blindness, paralysis and so on, though. Clerics have many buffs available to boost a party, have access to a handful crowd-control spells and are decent fighters of their own.

A beginner wishing to play a Healer can't make any mistake by choosing a Cleric and putting a minimum Wisdom score of 16. A good Wisdom score increases both the amount of spell points and the Difficulty Class of offensive spells (should the character opt for them at a point) while leaving room for fighting abilities (Strength, Constitution). Humans make good clerics since they are able to increase Wisdom through racial enhancements. More information can be found in starting a Cleric article.

The only limit of a Cleric healer is his pool of Spell points, putting the party in jeopardy if exhausted. Still, for healing the casting ability of Clerics is miles ahead classes described above. As Paladins, Clerics have access to Unyielding Sovereignty enhancement, further improving their healing prowess. The enhancement is very welcome at higher level, where a Raise Dead casting leaves a party member alive but with very low hit points.

Favored Souls
Favored Souls make up an interesting variant for healers. As Sorcerers to Wizards, they are able to cast much more often, but only from a limited selection of spells. Favored Souls have fewer spells than Clerics of the same level, must allocate spell slots to healing spells (they don't get Spontaneous casting) and access higher-level spells one level later than Clerics. Favored Souls are therefore ill-suited to circumstantial spells like Remove Paralysis or Cure Blindness, and devote their precious spell slots to more generic buffs and healing. When it comes to pure hit point restoration, however, Favored Souls are second to none. They have a large spell point reserve and gain twice as many Spell Points from items than other classes. At high level, the Quicken Spell metamagic feat gets more important as healing speed becomes a deciding factor; Favored Souls are able to perform "burst healing" longer than Clerics. Favored Souls have access to Unyielding Sovereignty enhancement as well.

Favored Soul builds are many. Their spell point reserve and max spell level is drawn from Charisma while the Difficulty Class of their offensive spells is drawn by their Wisdom, they are different than Wisdom-makes-it all Clerics. A Favored Soul will be strained characteristics-wise if she wishes to be a good healer, an offensive spellcaster and a decent fighter at the same time. Some Favored Soul healers drop Wisdom characteristic entirely to concentrate on buffs, healing spells and melee damage only. Others have a good Wisdom and Charisma for offensive casting, but raising both costs more enhancement points and makes the character a worse combatant. Neglecting Charisma is not advised since it is used not only for extra spell points, but also to determine the highest level spell the character can cast. 14 is a minimal score ensuring some access to high-level spells on the long run; lower values are possible only if you are really sure of your character development plan.

Multiclassed Healers
Any character may gain healing abilities by taking levels in a class with access to Healing spells. The same holds for enhancements and class feats: a Fighter multiclassing into Paladin for any reason will get some healing features of his second class (like lay on hands). Yet, as spell effects are linked to caster level, healing abilities acquired through these means are generally very low. At high level, such specializations are even less interesting but for different reasons: healing a 300-hit points Barbarian through Cure Light Wounds spell would take forever!

Common multiclassing include:
 * A Wizard taking a Bard level, gaining access to Cure Light Wounds spell (and UMD, armor casting, etc.)
 * A Ranger taking a Sorcerer or Favored Soul level to increase the size of his spell point pool.

Some synergies are possible that bring healing abilities to a character among other benefits; but the ability to cast Cure Light Wounds alone is not enough to justify a secondary class.