Do not, do not, do not boost your first character.
Boosts are handy for people returning to the game after an expansion or two. Boosts are great for people who want a new class at max level fast because their raid needs that class. Boosts are convenient for people who want to start on a different server.
Boosts are bad for beginners.
I’ve been watching this Newcomers forum for a long time. One of the recurring problems has been new people who boost, and are then lost and disoriented and alienated. They usually leave, or put their boosted character away and start from level 1 again. It always makes me sad.
It’s not just how to play a class at 120; it’s all the little things you learn along the way - geography, vendors, bags, travel, UI and addons and macros, XP, trade, communication, buffs, passives, roles. And trhen the systems you sign up for at max level are a bit elaborate anyhow, but when you have no grounding in the game before it, they really seem pointlessly complicated.
I’m glad you found a class to settle on - for now. Go for it, and enjoy it, but if you find your enthusiasm dropping as you level, don’t rule out other classes. We can’t really know what we enjoy until we try it. Most people who stay in the game - I would say 99% of people - play different classes on separate characters. These are called “alts”. Perhaps your main character will be a Hunter, but you will play a Demon Hunter on the side, or perhaps you’ll find you love Druid, and your Hunter will become an alt. This is all normal.
As for your realm question, by now I’d put it this way: if you’re going Horde, choose Draenor unless you have a specific reason not to; if you’re going Alliance, choose Silvermoon unless you have a specific reason not to.