Low level dungeons for a noob
Ascalonian Catacombs is a level 30 dungeons, but it’s a little too punishing for noobs. In my opinion Caudecus’s Manor (which is a level 40 dungeon) is the easiest for newcomers.
I recommend always doing dungeons with lvl 80 chacacters because the higher armour makes it much easier. As people have been farming Citadel of Flame (CoF) like crazy, especially path 1 and 2, you could start there considering how easy they are.
Just make sure that, whoever you decide to group up with, you’re clear about being new to dungeons and that you’re eager to learn. People tend to be impatient and kick quickly if you do stuff wrong, so a preemptive warning helps.
Far Shiverpeaks
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Do the story dungeons as they become available. They’re not easy or fast for new players but they are all achievable on level. You might as well leave the explorable dungeons for level 80 as they are harder. Since some people run the explorable dungeons repeatedly they want fast runs with everyone else at level 80, knowing the tactics, with good gear. You’ll get a better dungeon experience if you can go into the dungeons with friends/guild who can take time to explain stuff.