1) Start in tyria. You'll take longer than if you did factions/nightfall, but you'll get the smoothest introduction. If you're really impatient and can get help from more experienced players, try nightfall. I do NOT recommend factions as a starting campaign. Try prophecies first.
2)
http://pvx.wikia.com/wiki/Main_Page
Your builds depend on what you are fighting, what you are wearing. Sometimes terrain is taken into account. Sometimes you need to run a very specific skill set because someone else is running something that works well with what you might run. It varies insanely. Making builds is what separates many good players from inexperienced ones.
3) Weapons in GW are now based more on looks than stats. There is a finite limit to damage, and it's very easy to find those max damage weapons once farther in game. You can modify weapons to give them good mods as well. The most expensive weapons will be certain unique (green) weapons, which almost always have perfect stats and sometimes nice skins, or certain gold weapons modded nicely with nice skins. It's a very visual game. Same goes for armor, but it's customized to characters, so you can't sell it. Runes/Insignias are crucial for armor, and bought off of a rune trader (for the most part). Most runes don't exceed 1.5 plat, but superior vigor costs a whopping 20k. Can be frustrating.
4) There is an end to the
storyline of all the campaigns, but keep in mind this is an MMO. There is no "end" to the gameplay. It all depends on what you want to do. Many people switch from PvE to PvP after beating all the campaign, farming titles, etc, because it simply gets boring and eventually unrewarding. PvP is...hectic, changing, and requires a better mastery of skills than PvE does. Also requires some connections in order to do some serious PvP.
5) Just trudge through it. Once you get to level 20, ascend, beat the game, whatever, you feel better about playing the game to be honest. Just beat one campaign, and start the others, they have fairly creative story lines. Just don't skip the cinematics or you only get half the story =)
6) This is all personal preference. If you want other opinions, I'll tell you what I do. I generally sell all golds that don't have a worthwhile mod on them, or that don't have a "highly salvageable" inscription to the merchant. This assumes there isn't a good skin on it either. I salvage anything below that (generally) for materials. Armors, make sure the rune/insignia isn't worth anything before salvaging or selling. Generally pick up everything, decide what you want to sell and what you want to salvage. Materials are good for armors, weapons, and consumnables (Eye of the North only). You'll only get a better understanding of what you need through experience. You'll learn what is worth more.
Yeah, about inventory space, just take everything, and always replace a better item with a worse item. Since you can't always take everything, try to take the best possibly inventory away, no?
7) Mods. Hmm. This GREATLY varies with what you are doing, running, etc. A warrior using a general PvE setup with some damage and whatnot would want a +15% damage while health is above 50% inscription, perhaps a sundering (armor penetration) hilt and a fortitude (+health) pommel. Casters generally want energy and increasing enchantment durations or decreasing spell casting times. This you'll have to know specifically what to run. Most elemental mods are worthless. You can use them in some areas in a good way though. A fiery mod is great for the snowman dungeon accessible through Umbral Grotto.
I know it's alot to read through, and it's only my 2 cents. There's a ton of stuff to do in the game, as well as a ton to learn. I've had the game for 4 years and I still have never done certain dungeons/areas/quests that I would really like to, or obtained certain items I would like to have. Just play the game, have fun with it, and quit playing when you get bored and find a new game. Or get on with other real life things. Hope I helped at least somewhat =)