My girlfriend and I are noahide and we live together. We want to get married in December, but currently we refer to ourselves as boyfriend/girlfriend, not as married or engaged. By Jewish law, would we be considered married now or only when we are officially "married"? What about getting "officially" engaged, would that constitute marriage?
The Divine Code stipulates that in your situation, you would be considered as married if the public is aware of your relationship and your living together.
If a woman specifies herself for one man's cohabitation, but she is not considered fully married regarding the secular government's laws and assigned rights (i.e., they are only "living together"), then she is still considered a concubine in the perspective of Torah Law. This practice is permitted for Gentiles, and the woman is considered fully married if their shared-home relationship is publicly known. If another Gentile man cohabits with her, both are liable for adultery." (pg 436)
by non-jews, the initial sexual union (with intent to be in marriage) is in and of itself the "marriage ceremony." https://www.sefaria.org/Mishneh_Torah%2C_Marriage.1.1?ven=Mishneh_Torah,_trans._by_Eliyahu_Touger._Jerusalem,_Moznaim_Pub._c1986-c2007&lang=bi&with=all&lang2=en
Nevertheless, a public wedding ceremony is still appropriate.
I recommend you buy the Divine Code https://asknoah.org/books/the-divine-code
You can read more about this issue and most questions you would have about Noahide practice.
As far as I know Noachide law is very vague and under-developed in comparison with Jewish Halakhah. At least the opinion of the Rambam is that for a non-Jew marriage consists of (1) living together and (2) the community knowing that so-and-so is the "wife" of so-and-so. See Melachim 9:8 and Ishut 1:1. No specific ceremony is required. The big question is what "wife" means in this context, because Rambam doesn't seem to contemplate a situation where people live together but are not married. So if the society has a concept of marriage, and everyone knows that Alice is Bob's girl, but also knows that Alice and Bob aren't "married," that seems to be an ambiguous case, at least inside the Rambam.
In terms of engagement, just to give you the Jewish perspective what we call engagement today does not legally constitute marriage. In biblical and talmudic times there was a more formal "engagement" called kiddushin which, for some purposes, made the couple husband and wife (e.g., they would need an official divorce to separate). Today, however, engagement is really just a non-binding promise to get married. If the live-in boyfriend-girlfriend relationship is not marriage, I don't see how a contemporary engagement would be any different.