I’ve never bought alcohol

Every time I get invited to a BYOB party I bring a water or pop then tell people I don’t really drink. (Sorta true, I do drink just not often) Eventually someone will offer me a sip of their drink and after I sip a few peoples drinks someone will give me a full drink.

Pretty much the only reason I don’t just buy then bring my own drink is because I’ve never done it before and have no idea how as I don’t know what I’d like because I have only ever drank things people have handed to me / sips of others drinks in cups so don’t know what it was ( which is usually some kind of alcohol mixed with juice).

btw I’m 20 years old and the drinking age where I live is 19.