Update: Please note that this is just a general introduction for the topic. If you're looking for something more in-depth, I recommend this book. I recently received a copy myself (you can read my review here) and have to admit that this thing is in a league of it's own. It covers super advanced details on leveling, class progression, and making gold.
Leveling in The Elder Scrolls Online will largely focus on completing the main story while exploring the game and uncovering quests, rare mobs, dynamic content and dungeons along the way. As a Dragonknight, you should find that you will be able to do a great deal of AoE damage while fighting and that mobs should die fairly quickly. This means that you will be able to solo extremely well. Grouping with friends is another viable option, of course, and you may find yourself tanking in group environments even if you are not going the tanking route.
If you wish to level solo, you should concentrate on discovering as many quests as possible. This is where the majority of your experience will come from. Grinding mobs is one possible way to gain experience, but by combining killing with completing quest objectives, you will level much quicker. Make sure to uncover all of the hidden areas of a map before moving on to the next area. By doing so, you should discover most of the quests that are located at unknown landmarks.
Make sure to utilize a build that focuses on killing quickly and efficiently, but also one that provides a few options as far as survivability and mitigation is concerned as well as a small dose of crowd control or self-heals. You can easily change your build when you are finished leveling. By combining active abilities and passive abilities from all three Dragonknight skill lines, you can find a build that works well for leveling and soloing purposes.
Grouping With Others
When grouping with others, you may wish to have options for dealing damage as well as holding threat from your party members. Alternatively, you can slot a few support abilities and simply provide assistance that way. Flexibility is often key when grouping with others, especially against mobs that hit very hard. You may find that a ranged weapon is also useful, especially when fighting mobs that teleport during combat or that deal devastating AoE attacks in melee range.
Since ESO Dragonknight will be able to equip two weapons at a time as well as two sets of active abilities, you should experiment with different weapon and combat styles and see what fits you the best while leveling. Don’t be afraid to try something new. Fire staff Dragonknights, for example, are quite viable. The sky is the limit when it comes to creative builds that are well-suited for leveling.
For a lot more details on this, an in-depth leveling guide, and much, much more, check out the leveling guide in this site!