First, congratulations on reaching level 50 and finishing your class story!
Now it's a different game alltogether that starts.
you can use the group finder so that the system assembles a group from all the people in queue to do story mode, but most people will frown on you if you butt in unprepared. Don't hesitate to tell everyone that it's your first time, as they'll be more inclined to help you, but people will definitely look more favourably on you if you've done your homework before jumping into the battle.
A little guide on getting yourself geared up, from Dulfy (callsign "iwipe" on the forum)
A forum with strategies on the different bosses of the eternity vault operation
You may want to look for a guild too. Most (if not all) the content at endgame is best enjoyed in organized groups (pvp or pve).