The hierarchy of difficulty is fairly simple. The Craglorn 3 (Hel Ra, Sanctum Ophidia, and Aetherian Archive) are the easiest. AA is by far the easiest of those 3. After that, its the DLC 3 with arguably Asylum Sanctorum, Halls of Fabrication, and Maw of Lorkhaj in ascending order. That goes for normal and veteran difficulty. I have yet to do Sunspire, so I can't comment on it.

I agree with @Nestor, with the caveat that mechanics are more important in trials than dungeons. I'd say that the Craglorn 3 are easier than most veteran DLC dungeons, on normal or veteran. If you are around a CP300, have a competent rotation, a logical build, and decent sets you should be able to hold your own in the Craglorn 3. You certainly don't need the "meta" to do them on normal.

Run a couple PUGs or hook up with friends in a guild. That's the best way to learn the trials and have fun. It can be scary at first, but I think its the best content in the game. If you are gear farming, I'd recommend running with a guild as PUGs tend to be stingy.

Definitely have a mic. You will need to hear what to do. And don't be afraid to ask questions about stuff you don't know. The worst kind of trials player is the one who refuses to listen or learn mechanics and runs around wildly so the healer can't help him.

You can do normal at any time in any gear. For vet, I’d recommend at least 20k DPS for Craglorn trials (vAA, vHRC, vSO) and 300 CP’s (mostly for defense). Bump that up to a 30k DPS minimum for any vet DLC trials (vMoL, vHoF, vAS+0, vCR+0, vSS). Beyond that you’d be looking at vet hard modes, which you want about 40k DPS and max CP’s for. Of course more DPS just makes things easier, so 50-60k has a massive effect on smooth, quick clears.