Mostly just at home. My dad is American and mom is Greek, so they each spoke to me primarily in their native language and I grew up speaking both natively to some extent. The Greek side's grandparents also lived with us for about a year when I was aged 3-4, which I think helped a lot. I'm told my Greek was better than my English up until I started going to school, and then my English rapidly got better and I started using Greek less often even with my mom— she'd speak to me in Greek and I'd often reply in English.

I did take some lessons from the local Greek Orthodox church's after-school courses, but most of the kids spoke less Greek than I did (many were 2nd or 3rd generation Greek-Americans) so I don't think I learned a lot there as far as spoken fluency goes. I did learn the basics of reading and writing from there, but this didn't seem interesting to me at the time so I didn't really put much effort into it. Now as an adult I put a little more effort into reading Greek newspapers now and then, but I still approach it as mainly an oral language and read very very slowly due to lack of practice.