The things taught by the Great and Holy Councils, mostly.
The thing is, and it’s generally found to be a most unsatisfactory answer to those I’ve given it to before but it’s true, you just pick it up. You ask questions and learn the ‘mindset’ which helps you answer questions or situations you find yourself in. It works, probably because of the Holy Spirit. You just sort of come to know what is absolute law - like the Divinity of Christ, the title Theotokos, Confession forgiving sin, and what you can have some leeway on - like the Lenten fasts, prayer rules, and toll houses. You learn that the Lenten fasts are something you work out with your priest but that the fast before Communion is very, very rarely up for debate. That you can have varying opinions on the judgement and such, but that you must pray for the dead. That you might only Confess once a month or so, but that you have to believe God forgives sins.
But there is very little difference between what we do and say and what we believe, and what we believe is generally so interwoven and connected that distinguishing “this is necessary, but this isn’t” isn’t really something we do. We’re more likely to agree than not, and more likely to regard certain things as having some wiggle room for interpretation rather than being ‘necessary’ or ‘dogma’ or ‘little t-tradition.’ From the outside it sounds like a huge mess, but when you’re in it and living it it makes complete sense. Ultimately, though, it like this: We baptize infants because you must believe Baptism is a Holy Mystery for salvation and entrance into The Church. Same with Chrismation and Communion. We address Mary as the Theotokos because she is the Theotokos. We conduct reverent Liturgies because God is worthy of reverence, and describe Him in apophatic terms because He is the Indescribable. We do metanias before everyone in our parish on Forgiveness Sunday because everyone is an image of Christ, and we sing that the Virgin is the burning bush and the ladder to Heaven and the eastern gate because these are all pre-representations of her. To learn what we believe you look to our hymns and actions and ask what they mean.