This site is one of the best resources to get you up to speed on Android in general, though there are others.
A great refresher or quick-look at the bigger components of the whole Android ecosystem:
Android A to Z: An Android Glossary | Android Central
You already know that the forums here kick ***, as long as you can navigate through the fanboys and the silly, yet heated, arguments. Unless you like those and then they can be very fun. Pay close attention to the stickys at the top, as I'm sure more will be added as this device is released worldwide:
HTC One - Android Forums at AndroidCentral.com
Not an Apple guy personally, but I am in the minority even in my own house with that. The new version of HTC Sync is supposed to be great at all of the iPhone data transfer you may need. The FAQ'a on the HTC site are pretty informative:
HTC One can sync from your iPhone backup - Crave - Mobile Phones - CNET Asia
HTC Sync Manager
Easy way to sync your music, though it won't work for DRM-protected files:
How to sync iTunes music to Android devices
I don't use iTunes so I really don't care, but mp3 files are mp3 files. If you bought your music through iTunes, that's a different story - there is the option to go DRM-free w/ iTunes music, for a price. There are some programs out there as well, but they cost money and I'm not sure of the results you'd get. Others here would be more capable of speaking to that. I do know that you can always go old-school and burn those DRM-protected songs to an audio disk and then rip them back to your computer as mp3 files.
Good luck and welcome to Android!