Well, here's a reference for everything you have access to now that you're 50.

Your main objective is to start raising your iLvl. Tomestones are the easiest source of that for body gear, which you'll gather passively (as well as the drops you collect doing dungeons) and your A Relic Reborn quest is best for getting yourself a weapon (I suggest completing it and then doing the step to upgrade it once more than that for sure - it's a relatively painless i90 weapon, which is still sufficient for about 80-90% of postgame content; by the time it comes time to replace it you'll have access to methods to do so easily).