First of all, Factions isn't a mistake. It's a very good game, although most people started out somewhere else and learned to like that campaign better. Personally, I like Factions over all of the other ones.
Secondly, I'd say go with whatever profession appeals to you most. People can learn, and I don't think monk is one of the harder professions to learn.
Really, at this point you don't need to worry about classes at all. Just pick whatever you want, and the secondary that appeals to you best. Try them all out, if you want. Later on in the game you get a chance to change your secondary to anything else, and switch it whenever you want to anything.
I find the best way to learn to play GW is just to go out and try things, rather than worrying if you have best class/best armor/best items right from the start.

There's nothing you can do to completely mess up your character, unless you're going for certain titles (and those I don't suggest until you've gained some gameplay experience). And if you want, you can even create multiple characters so you can switch between them if you suddenly feel like you don't like what you created. Also, characters can be deleted and then remade, so GW allows for a lot of freedom when trying to find a class that suits you the best.
Just remember to not sell green items, black/white dye and some rare materials to the merchant and you're set to go. You can usually get a better price for them from players.
Primary quests lead you along the intended storyline. When you get to a town with a mission available, you will probably not have any more primary quests to follow until you've done it, so your list will appear empty. Also, a button with the word Enter Mission will appear on your party menu. Normal quests are just quests that you get exp and loot from, usually nothing else.
Hope this helped.
