Q: About 1st dungeon and classes.
First Dungeon: Dungeons have been around since day 1 and a lot of folks just try to speed through it since there has been no mechanical change to the dungeons. As in the dungeons you see today were laid out almost identically on the first day the game came out. So, if you’re wanting a more slower pace run or with explanation you’ll want to join a group in LFG that lists that or create your own group (if you can) specifically asking for a walkthrough. No shame there, a lot of veterans don’t mind helping the newer folks since we were all once new to the game. Once you know what to look for you’ll be on your feet a lot more.
Based on your description I’m going to guess that you were doing the Ascalonian Catacombs dungeon in what’s called Explorable mode (the spider boss gives it away). As such, that dungeon is design for a level 35 character. Given that you’re level 30 and probably don’t have the best gear? No surprise you got trucked. Players beyond level 35 get their stats and gear scaled back to 35, but they retain access to their skills and Specializations/traits. So they have more tools to work with and slightly higher stats even after being scaled.
Re: class selection? I’ll start off by saying that you’re kinda limited in your customization until you hit 80 as you’re locked out of certain traits and skills until then. Even more so if you’re 80 but haven’t unlocked your Elite Spec. With that in mind…
Core Necros only have access to Staff until you unlock Reaper. Staff is an okay ranged weapon that does pretty well with Power scaling, but is awkward to use due to marks causing conditions. Thus, staff isn’t really a good power weapon nor a great condi weapon. Greatsword is mostly Power based, but with a handful of traits can also be used as a condi weapon. For the most part you’ll want to learn being comfortable in melee range if you plan to play PvE as a lot of boons/buffs you get from other party members have a small range, and it’s much easier to buff a team that’s clumped together rather than trying to hit everyone that’s spread out. Though for open world you can pew pew with staff or axe just fine.
Re: Ranger. One of my personal favorites. Great offensive buff-er when in “heal mode” but they are highly aggressive with damage when in a condi build. Power builds fall short in PvE compared to other classes though. Same advice as necro though, you’ll want to get used to being on the front lines even though you can range.