Almost Level 50 and Terrified

[Dement]

Once you hit 50, get your AF body and finish out your job quest line for the final skill. Then you'll want to finish out your main storyline which it sounds like you'll have to do through duty finder. Be advised that doing this through duty finder will be a crap shoot depending on how invested you are in the story. There are a lot of cutscenes that pop up in the final two dungeons and veterans running those dungeons for their roulette sometimes are not very pleasant when it comes to new players watching all the cutscenes. You'll have to make a choice if this happens on if you want to watch the cutscenes later in the inn room or if you absolutely must see them then and there you will probably miss fights as the others will go on ahead of you. Be prepared cause you could also get flamed for watching the scenes as they go from some of the more obnoxious players.

With your main story complete through the quest "The Ultimate Weapon" a lot of endgame stuff will unlock. You'll want to start your relic quest (from Gerolt in the NE corner of North Shroud). To start it, I believe you have to have cleared the lvl 47 dungeon "Aurum Vale". The relic quest will entail getting a base item from crafting (or buying on the Market), melding it, killing some open world level 50 monsters, completing the Amdapor Keep dungeon, killing Dhorme Chimera and Hydra. You'll also need to unlock Ifrit HM (Hardmode), Garuda Hardmode, and Titan Hardmode as they all need to be killed in that order to get your relic weapon. Upon completing you'll get a nice shiny ilvl 80 weapon

You'll also want to start focusing on your ilvl from your gear (the number that appears above your character in the view character screen) as there are ilvl locks on most dungeons. The entry level dungeons are going to be Wanderer's Palace and Amdapor Keep (Which you need to clear for your relic anyway). These may drop some ilvl60 gear that you can use but more importantly they'll drop mythology tomestones and soldier tomestones. You can get unlimited mythology tomestones but are capped at 450 soldiery tomes per week. You use these tomes to buy ilvl 90 gear (mythology) and ilvl 100 gear (soldiery) from a tent in Mor Dhona (I think the NPC is named Auriana or something similar. She's in the northern part of the town and will typically have a large crowd around her).

As you gear up you can open up the next set of dungeons which is Pharos Sirius, Copperbell Hardmode, and Haukke Manor Hardmode (all with ilvl70 gear and myth/soldier). You can also unlock the first Crystal Tower dungeon: Labyrinth of the Ancients which drops ilvl 80 gear and after you're comfortably a bit above ilvl 70 with your relic weapon you can unlock the first Binding Coil of Bahamut which also drops ilvl90 gear.

Once again, when you have a bit higher gear to enter, you can unlock the Lost City of Amdapor, Brayflox Longstop hardmode, and Halitali Hardmode for your myth/soldiery drops. Once you have all of the dungeons to here cleared you can do Hard Roulette which places you into a random dungeon and gives you a myth/soldiery bonus once per day.

Your next set of dungeons at an even higher ilvl is going to be the current content which consists of Hullbreaker Island, Tam Tara Deepcroft Hardmode, and Stone Vigil Hardmode

Should also note that the story continues along leading into a fight against Good King Mog, Leviathan, and now Ramuh as well so you can continue along your story to unlock these fights. Once you've cleared the story version of these fights, Urianger in the waking sands will allow you to unlock the EX (Extreme) versions. You'll need to kill Ifrit Extreme to challenge Garuda Extreme and clear that to challenge Titan Extreme. Additionally, you can fight Mog Extreme and Leviathan Extreme without killing Ifrit/Garuda/Titan and you need to kill Mog and Leviathan to fight Ramuh. (You'll probably want to be ilvl85 or higher before really trying to tackle extreme primals).

That should give you a basic road map on what you need to do once you hit 50. Some stuff may be slightly off as it's been months since I've been there. The only thing I didn't delve into is how to make your relic stronger but a bit of research and you should be able to find that. (Look into Atmas). Or you can flat out get a new weapon drop from the extreme primals or Turn 5 of the Binding Coil of Bahamut. (both relatively difficult)

When it comes to learning these things, you didn't really miss a point in time when it was "okay to fail", even brand new fights people are asking for experienced players within a week. Look for learning parties in the party finder, get friends from FC to help you, research the fight online, and set up a learning party yourself if you have to.