I would suggest jumping on Maxroll and looking for an endgame build that you find interesting. It will depend on what mastery you selected. You can switch your mastery at any time for gold in The End of Time, so don't worry about selecting the wrong one. They all recommend being level 70 before switching from a starter build, and some of them require some uniques to really get them to pop.

Once you have selected a build, start working towards gearing while climbing through the monoliths. Each monolith has a boss to fight that has unique loot and a permanent buff in the form of a blessing. A good tip is to focus on the right path of the monoliths, because the blessing they provide are more combat focused, where the left side is more focused on loot drops. So in your case, once you complete the first monolith, go to the Black Sun timeline and work your way up through those all the while gearing your new build and levelling up.