Things have changed in the years following fi12's answer.
The Valyrian Wiki (aka. The Tongues of Ice and Fire Wiki)1 is still the main resource. However, David Peterson (DJP) recently uploaded his work on a Reddit AMA. This contains:
The Academy of High Valyrian is still the best series of videos about HV, although if you want the pronunciation (and more than the absolute basics), DJP himself made the audio (as well as the content) for the Duolingo course.
The forum is no longer in use, as we've instead migrated to a server on Discord named Valyrio Udrir. There's also a Facebook group, Duolingo High Valyrian Learners.
In addition, I've also made a few resources. They're not quite as up-to-date as the Wiki, but they're not far behind. I will update these when time permits, but that could be a while away:
- A Dictionary, available in word, pdf, access database and csv formats.
- I used this to create a Decks/Memrise course. (there are a few others available, although they are much smaller in size)
- This was also used as the dictionary in a Discord bot called Merlin2.
The links I've emboldened are the ones I view as the most important.
As a bonus, if you want to read through more detailed explanations and some details about how the language was created, DJP also made a series of blog posts over the years, where he still holds the occasional Haiku competition.
1 For full disclosure, I'm one of the editors
2 Created by Sorcerertech, also has a Klingon dictionary