Most people end up making several alts and maining one character, so you could always try out several classes before settling on which you'd like to call your main. Keep in mind that the diversity/differences between classes becomes more apparent at level ~30 when you unlock your elite skill and traits.

To answer your questions about necro: They can't use scythes, although dagger/dagger is decent in PvE. Generally people tend to run dagger/focus and dagger/warhorn though. Necro isn't exactly in a great place in PvE at the moment as everything they can do, other classes can do better... This may change with the expansion due to class specialisations.

Warriors and guardians are the most demanded classes when it comes to dungeons and group content. Warriors have decent DPS and can utilize banners to increase overall group dps, and also have an emergency rez if it is needed. They have some pretty amazing mobility and have the widest selection of weapons at their disposal. Warriors are considered to have the lowest skill floor and skill ceiling; they're great for newbies to learn and are useful in groups, but there isn't really much challenge to playing them well.

Guardians aren't only a support role, that simply comes as a side effect of playing the class properly. They can inflict blind and vulnerability like a champ, stack might, put up projectile-reflects, cleanse conditions en-masse, and apply passive heals and buffs to allies. Their solo DPS is also great, assuming you don't just camp staff. A good guardian will be welcomed by any team, they really do bring a lot to the table. Slightly harder to play than warrior with a higher skill floor and ceiling.

I personally main a thief and find it the most enjoyable class to play, with guardian being a close second. Lots of blinds, evades and stealth, the highest single-target DPS and some fun weapon combinations. Shadowsteps are also fun and making getting around a fair bit easier in some cases. However, it doesn't really grant as much to a group as say, a warrior's banners or guardian's general utility.

As for casters, every class has at least one melee-distance and ranged weapon option. Elementalists and necros are given a bit of leeway in terms of their melee options as they have a short range of ~300, so they can still kite mobs and avoid a bit of damage. Ranged weapons do less damage than melee, but they do have their uses. They're all quite similar in terms of how they feel in combat though, so playing a scepter/focus guardian or longbow warrior doesn't feel terribly different to using a greatsword.

I'd honestly suggest just picking a class that you find looks fun, and just see how it goes. Different people prefer different classes, and you won't really know which is your favourite until you try.