Solo Advice

If you remember very little about the game I would recommend starting in Prophecies. The slower learning curve will give you a much better opportunity to learn how your profession works and general PvE things. Heroes are not necessarily a boon - their builds can be customised so they can potentially be much better than henchmen, but they can also be worse if you don't have many skills unlocked or don't understand how those professions work (which is overwhelmingly likely if you haven't actually played them). Hench bars are generally pretty good nowadays so you really shouldn't look at heroes as a big priority.

Play whichever profession you want. If you give us an idea on what things you're looking for we could give you specific recommendations, but choosing solely based on which is the most powerful is inherently stupid. You're meant to be enjoying yourself, remember? One thing I will stress though, is that there are no "bad" professions. ArenaNet gave up on nerfs in 2009 or so, so all skill updates since then have been buffs, leading to a massive surge of power creep. You'll see people say things like Paragons and Rangers are bad, but they aren't. They just aren't game breakingly overpowered like Mesmers or Rits. You can comfortably clear the game in Hard Mode with any profession.

Other advice? Don't skip content/rush ahead. Take your time to immerse yourself in the game and learn as much as you can.

they can be, however, if as Marty says, you have very few skills unlocked they will be not very useful. They are more for people trying to do certain builds for their whole hero group (they are heroes, they just look like your characters).
If you still have an old account and old characters you could just finish up what you missed (or at least keep the old characters for later). If you are starting with a new account then I would start at the beginning as Marty suggests.
As for what profession, eles are pretty easy to play as are warriors, rangers are safe as they have a higher armor rating (at max level), monks and mesmers are a bit harder to play (though my favorite prophecies character is my monk----), necros have some good points too, and are quite versitile. If you are a melee type then you would also enjoy sin or dervish or even possible paragon.....all depends on what you like.

Guilds are absolutely still around. They're the basis for most player interaction nowadays - people don't typically group up for random missions and such because it's just easier to do that kind of stuff with heroes, plus there's more of a focus on high end stuff given the age of the game. Outposts may seem empty but that's because people are out in instances doing things. Being in a guild will allow you to be connected to those people, and potentially get together to do stuff.

Also, playing with heroes and henchmen is shortened to "H/H". Playing "solo" means being a one person team.

Also also, if you're not already there, switch over to the American districts. Pretty much everyone is there.

people play at all hours of the day...so no worries there.
There are still plenty of active guilds (look at our guild section and see if anyone is recruiting right now), I know there were a bunch of guilds that were helping out new people--or willing to do all the beginners stuff...cant remember who they were now (ok, can barely remember my OWN name right now---really do need to sleep a bit more :P)