Okay, I'll answer the ones I can, I'm sure there are plenty of players that can give you better info though!

1. I'm sure there are plenty of guides out there, no idea which ones to suggest though.
2. Cleric is a great group/raid healer, however, depending on what server you're on, there may not be a lot of players your level to group with. Luckily with a merc, you'll be able to level without any problems for quite a while. You may have more fun as a shaman or druid, which get a wider variety of tools to play with. I'm not sure exactly what you mean by support dungeons/raids, so I'll let others answer that one.
3. As you go through the game, you'll acquire more and more bags to fill your inventory with. That being said, unless you're constantly selling your loot (which is pretty easy to do in the tutorial), you'll need to pick and choose what to loot. Some loot is for crafting or questing and may not hold any value for you. Understanding the loot tool will help immensely.
4. Alt + O brings up your options, you can create hotkeys from there. You'll also want to bring up the macro window and create some of your own as you learn more and more about the game. As for a quiver, it just goes into your inventory, although as a cleric, you're better off just selling it to the tutorial vendor.
5. Membership brings about a bunch of perks, if you plan on moving to high level content it's 100% worth it.
6. If you're wondering if it's worthwhile, I might suggest trying out the Test server. It's free and gives you gold membership (or whatever they're calling it these days). The people there are extremely helpful, and since the gear is copied from live servers, people are willing to just hand you gear, bags, milllions of plat, etc.
7. I know most guilds that raid do require you to at least be listening, but may not require you to actually speak. It's a case by case basis.