Seems a bit daunting, so much content and i just noticed the class I would like to try out is only available at 50? Seems a bit weird, but I don't know anything so....
The good news is this is actually a pretty good time to get started. The endgame at 60 is on the thin side, but the A Realm Reborn content is pretty solid and fleshed-out, and they've removed a lot of the grind from getting to 50 the first time.
Jobs introduced in the expansion require you to have cleared the main story of A Realm Reborn to unlock them, since they're based in a city you can't get to until you're in expansion content.
On the other hand, the game is structured such that you generally want to do at least some leveling of multiple jobs to unlock cross-class skills you'll use later. So if you want to main Dark Knight, you can do your initial 1-50 on Warrior or Paladin and learn how tanking works in this game. Then at 50, unlock the new job and start leveling it from 30. 30-50 doesn't take long at all. Astrologians would probably be best served getting to 50 on White Mage first, and Machinists share a role with Bards.
Does this have a healthy population?
It's not bad. The endgame is in a weird state right now, but that won't affect you for quite a while. Different servers have different activity levels. Ultros, where GAF is, is lower activity but for dungeons and things it matches groups across servers so that doesn't matter. GAF FC itself is still okay activity-wise.
How are queue times for dungeons?
Role dependent; they have I think made changes to give priority to people trying to do a dungeon for the first time so the queue time shouldn't be too bad for mandatory story stuff. Tank and healer queues tend to go faster for most content, but DPS queues during peak hours are usually tolerable.
Does the gaf guild accepts newbies
?
Newbies are sort of the lifeblood of the FC. We have a lot of people join up when brand new. Some stick with the game, some don't. But we have no standards so you'd be fine.