The info below is a bit out of date as it refers to the beta. If you happen to be looking for an up-to-date reference for your Dragonknight, I suggest this manual. It includes some killer advice on leveling up, builds, skill rotation, and gold farming.
Tanking Dragonknights will want to focus on holding the attention of enemies faced in The Elder Scrolls Online as well as reduce the amount of damage they take through mitigation and avoidance. The Dragonknight skill line most suited for tanking is the Draconic Power skill line. This is where the majority of your skill points should be located.
Draconic Power Skill Line:
For active skills, you will most likely want to slot Spike Armor, Inhale and Dragon Blood. These provide you with mitigation tools as well as self-heals and ways to deal reflective damage. Dark Talons and Reflective Scales are more optional, but still viable.
For passives you will want Iron Skin, Burning Heart, Elder Dragon and Scaled Armor. All of these are extremely valuable for tanking. The Draconic Power Ultimate is an AoE leap ability that is worth taking.
Ardent Flame Skill Line:
While the Ardent Flame skill line is primarily focused on DPS, there are some nice active options here for tanks. Fiery Reach is useful for pulling casters to you. Fiery Breath and Searing Strike are solid DPS options if you’re looking to provide more damage. The passives in this skill line are entirely optional.
Earthen Heart Skill Line:
The Earthen Heart skill line is more focused on support abilities which makes the majority of them optional, but there are a few worth making note of. Obsidian Shield is a powerful active ability that works as an absorption shield for yourself and your group which is a perfect skill when you match up with the powerful cc class eso sorcerer. Ash Cloud is also useful as it will lessen the amount of damage you take.
The Earthen Heart Ultimate is very good for tanks as it limits the amount of incoming damage and deals a fire-based DoT to enemies around you. It may be worth using instead of the Draconic Power Ultimate.
Weapon Skill Lines:
The one-handed weapon and shield skill line will have the majority of skills you want as a tank. Puncture is your taunt ability. Low Slash helps mitigate damage. Defense Posture will come in very handy. Shield Charge is a charge ability that may be useful.
For passives, Fortress, Sword and Board, Deflect Bolts and Battlefield Mobility are all extremely useful.
Other weapon skill line combinations may include the two-handed line or the dual wield line. You can swap to these weapons during moments of low damage or when first starting a fight to quickly build threat.
The info above is a bit out of date as it refers to the beta. If you happen to be looking for an up-to-date reference for your Dragonknight, I suggest this walkthrough . It includes some killer advice on leveling up, builds, skill rotation, and gold farming.