How do I betray?Follow

Start here. This tells you how you betray from Kelethin. Once you're in Haven, there's a quest that'll send you to Neriak, Freeport and I think Gorowyn so you can get an idea where you want to go. But since you've decided already on Neriak, there are 2 NPCs that are right outside Neriak that will give you faction quests. Once you're ok with Neriak, you can then become a citizen.

I'm not sure about your spells if you stay the same class, but yeah, if you're going from say, a conjurer to a necro, your spells would be reset. That's what I did. And I started my betrayal around level 15, so it wasn't so bad that I had to get all new spells.

Same as Nad. I betrayed a Brigand to Swashbuckler just to see what betrayal was about. It's dead easy, but earlier is better - before you get used to doing things a certain way, as well as to cushion the impact of resetting all your abilities.