Starting a death knight

In order to be able to start a death knight character, players must have a level 55 or higher character on any realm. Players may have one death knight per realm, per account, but when the character is first created, it is required to complete a series of quests. By doing these quests, players learn how to bend the power of the death knight to their own will, and after they have mastered that, the death knight is then capable of going to any other area.

Unlocking the death knight
For any players to unlock the first hero class, the death knight, the player must have a character level 55 or above on any server. Players are limited to creating only one death knight on each realm.

Previous information also says that death knights may not be created on newly formed realms until incoming character transfers are allowed to that realm, unless the account has a character which has leveled up to 55 on that particular realm.

Creating the death knight
Any race is available for the death knight. When created, the death knight begins at level 55 with item level 60 gear,, as well as a few spells and abilities already learned. Death knights start without any talent points available, and gain their missing 46 talent points through the quest chain. As any other class starting out, there is a beginning series of quests that bring the character into the game. Their first sword is the Runed Soulblade, awarded for completing the second quest in the In Service Of The Lich King quest chain.

Death knights come in to being with First Aid at 270 skill.

Unlike other classes, death knights start without a Hearthstone. Instead, they start with a Scourgestone (death knight), which has the same functionality but can only be set to Acherus. They get a Hearthstone to replace their Scourgestone once they talk to an innkeeper after the end of the introductory questchain.

Death knights spawn in a necropolis in an instanced version of the Eastern Plaguelands, called Acherus: The Ebon Hold, which includes a dedicated questing area and class trainers.

For the actual quest chain, visit Death knight quests.

At the conclusion of the quest chain, death knights wind up with the item level 70 gear seen in the paper doll frame below.

There are options for the cape: And options for the weapon:
 * Sky Darkener's Shroud of Blood, Sky Darkener's Shroud of the Unholy or Shroud of the North Wind
 * Greataxe of the Ebon Blade or Greatsword of the Ebon Blade

Race
Many players like to choose the race with the 'best' racial traits and abilities for their class. Here is a brief overview of how the racials look from a death knight's perspective.

Alliance

 * Higher reputation gains. Able to remove snares and crowd control effects from self. Expertise with swords and maces. Better stealth detection.
 * Can remove bleed, disease and poison effects while briefly increasing armor (and attack power, with Bladed Armor talent). Expertise with maces. Small chance to avoid frost spells.
 * Able to vanish if things are going badly. Less delay from dying. Chance to avoid melee and ranged attacks as well as nature spells, useful for tanking.
 * Able to remove snare and root effects from self. Small chance to avoid arcane spells. Bonus to Engineering skill.
 * Increased chance to hit for self and group. Able to heal self and others. Small chance to avoid shadow spells. Bonus to Jewelcrafting skill.
 * : Able to increase running speed for a short time. Reduction of time from curses and diseases. Increased Critical strike. Bonus to Skinning skill. Starts with a "mount".

Horde

 * Able to increase attack power. Expertise with axes. Stronger minions. Shorter duration on stuns.
 * Able to remove fear, sleep and charm effects. Better post-combat self healing when fighting humanoids or undead. Small chance to avoid shadow spells.
 * Able to stun nearby enemies. Larger health pool. Small chance to avoid nature spells. Bonus to Herbalism skill.
 * Able to increase haste. Shorter duration on snares. More damage versus beasts. Faster health regeneration (a minor benefit due to all your life stealing and self heal effects).
 * Able to silence nearby enemies and gain runic power from it. Small chance to avoid all spells. Bonus to Enchanting skill
 * : Always able to buy items at lowest price. Starts with a damaging ability and a movement ability. Has a Mobile bank. Bonus to Alchemy skill.

In the end, race is a rather minor consideration. If min-maxing, Goblins are the strongest Horde race for DPS and Tauren the strongest for tanking. For the Alliance, Night Elves and Dwarves both make good tanks and Worgen are best for DPS. But go with whatever race you feel like, whether for its racials, its looks, its personality and flavor, or any other reason you can think of.

Professions
Death knights start from scratch with their professions and it can be tedious to build them up. However, if players are willing to do so they can reap the benefits of whatever professions they choose.

Primary

 * Alchemy: Death knight players can make their own potions and elixirs, made even better through mixology. This profession has a good level of demand since most of its products are consumable and the buffs stack with magic and well fed effects. Mixology bonus is equivalent to the bonuses from the other professions.
 * Blacksmithing: Being able to make plate armor and melee weapons should appeal to a heavy melee class. Unique gem sockets in gloves and bracers; allowing for a good deal of customization.
 * Enchanting: Convenient enchantments should appeal to any class. However, death knights should never enchant via enchanting their own weapon, as runeforging is strictly superior. Enchanter-only ring attacks (+60 stamina or +80 attack power).
 * Engineering: A vast array of gimmicks. Choice between engineer-only armor boost to gloves (excellent for tanking) and an on-use haste boost (decent for DPS).
 * Herbalism: Lifeblood furthers self healing (nice while leveling, useless in endgame). Fuels alchemy and inscription.
 * Inscription: Gives players a convenient source of glyphs and scrolls, along with shoulder enhancements at high levels. Saves having to do the Sons of Hodir grind in Wrath. The itemization on the shoulder enhancements is less than ideal, but not terrible.
 * Jewelcrafting: Convenient gems are good, and there are some rings, necklaces and trinkets that a death knight might like. Special JC-only gems, giving a customizable bonus to anything.
 * Leatherworking: None of the BoEs will do death knights any good, but they do get a nice stamina or AP boost to bracers, and cheap leg enhancers.
 * Mining: Toughness +60 stamina. As good as any other profession for tanking; useless for DPS. Fuels blacksmithing, engineering, and jewelcrafting.
 * Skinning: Master of Anatomy increases critical strike rating (okay for DPS, bad for tanking). Fuels leatherworking.
 * Tailoring: This offers death knights bags and clothes for role-playing, but not much else.

Secondary

 * Cooking: Well fed buffs are nice.
 * First Aid: Death knights do have self healing, but it's limited, so keep up with first aid. Conveniently, death knights start with 270 first aid and the ability to make Runecloth Bandages, which are level-appropriate.
 * Fishing: It's slow to level up, but it makes cooking and some primary professions easier, and there are achievements to be had.

Blizzard regarding the death knight creation
Bornakk, a CM on the US forums, wrote, back in 2007 before the launch of Wrath of the Lich King:

The idea seems to be a bit differed, as the Death Knight doesn't just realize on his/her own that he/she doesn't want to go through the evil acts, but is fighting for the Scourge unwillingly and the Lich King reveals that their life is meaningless, and during the time, most of the Death Knights (except for Orbaz Bloodbane, who flees) regain their will from their life.

Trivia

 * It is stated in-game by Master Siegesmith Corvus that he forged the Questline complete gear.