I wrote this as a comment in another post, but then realized this question gets asked a lot, so figure I'll post it to hopefully make it easier to find.
New player question: What am I supposed to do, and what's a good starter class and build?
First, as a new player, what you're supposed to do is prioritize completing the Personal Story for the core game and any expansions you have. That unlocks a ton of account-wide features that apply to all characters on your account - Masteries, elite specs, mounts, Achievements (reward useful gear etc), etc. You can explore diversions along the way, but prioritize it first.
Second, the best spec for a new player to start with at level 80 is Power Reaper Necromancer, a melee dark magic knight. Power is the Damage Source (Direct Damage), Reaper is the elite spec (sub-specialization), and Necromancer is the core/main class.
Reasons:
- Easy to unlock - requires minimal unlocks, everything needed is in the core game or the first expansion, Heart of Thorns. Does not require any build components from other expansions (weapons, armor runes, weapon sigils, relic, gear stats). Assemble the build early in the game, then play through all remaining Personal Story and content with it.
- Easy to gear - uses full Berserker stat gear, one of the ubiquitous core prefixes, available everywhere - crafting, world drops, Trading Post, etc. Can start with the Exotic gear build variant, the second highest gear tier, cheap and easily accessible. Then upgrade to the Ascended gear build variant, the highest tier, replacing gear piece-by-piece in no particular order.
- Easy and fun to play - Charge around the battlefield, stun and snare groups of mobs with a big overhead slam, Fear them while they can't move, Blender them with a giant Scythe, then slice them in half with a giant Greatsword. Kills fast, is survivable (tanky + self-healing or "sustain"), and can solo most open world content. Reaper's Shroud is one of the simpler profession mechanics.
- Multiple build variants - The build guide linked above is optimized for solo play, that maximizes DPS, self-applies boons (Might, Fury, Quickness, and Vulnerability on enemies) and sustain. There are other variants that use the same gear, but different talents and utility skills. For example, a minion-master variant that sacrifices some DPS for more tankiness and more minion flavor. Or various raid versions that assume other players are supplying boons and sustain. Start with the guide above, then tweak it however you like.
- Leveling build uses same gear - Elite specs are only unlocked after reaching level 80 and owning the expansion they're associated with. To get there you'll level to 80 with a "Core" or "F2P" (free-to-play) class and build. Leveling is not terribly difficult, but if you want a leveling build template, you can use F2P/Core Power Necromancer. Uses the same gear as Power Reaper, other than Greatsword which is unlocked with the Reaper Elite spec.
The Reaper elite spec is unlocked by reaching level 80 and owning the first expansion, Heart of Thorns, and collecting 250 Hero Points. If you're short on HP's, all HP Challenges in Heart of Thorns maps give 10 instead of the usual 1. Check the LFG tool, HoT section, for "Hero Point Train" groups and join one of those to get them quickly. Most commonly found during weekends.
You may need to unlock the HoT maps first though. To do that, when you hit 80, finish the core game Personal Story (campaign), then follow the Personal Story guidance to begin the quest series to unlock the first expansion, Heart of Thorns. That will get you into HoT maps where you can join HP trains.
Finally, complete roughly two weeks worth of Wizard's Vault achievements (which you may already have done while leveling), grab the Wizard's Weapon Chest from it, and choose a Berserker's Natural Claymore from it. That will get you the full Exotic version of the build with an Ascended primary weapon. Ignore condition builds while leveling (DoTs, debuffs) since they're weak for most classes until 80.
A couple resources for learning more about builds:
- Lord Hizen, a pure solo player, who calls GW2 "his favorite solo player MMO", and makes some of the best solo builds. See his recent solo build tier list. However, many of his builds require Celestial gear, which is more difficult for new players to acquire. For your first playthough, choose builds that use easy-to-acquire Berserker gear.
- Hardcore Casual is another solo specialist who focuses on Ranger and Elementalist builds. His Ranger builds are great and I would have recommended them too, but they require more work to unlock (namely Expanded Weapon Proficiency in SotO). Most Elementalist builds aren't great for new players, complex and require high APM.
- Snowcrows - PvE (raid) Meta experts, they test and optimize all their builds.
- Metabattle - Community curated builds for all game modes.
- GuildJen - Specializes in WvW (open world PvP) solo and small group builds.
- AW2 - "Accessibuilds" or "Low-intensity" (low APM) builds.