writersoul: Rambam Hilchos Talmud Torah 1:13. It’s quoted in Shulchan Aruch YD 245 (maybe 246).
That said, the Drishah on that Siman in the Tur says that if a girl volunteers to learn on her own then it’s totally fine. While this P’sak is not publicized so much, most seem to assume like it. Some extend this Drishah to mean that if a girl volunteers to learn you are then allowed to teach her without any parameters. Many do not like this extension of the Heter. But most knowledgeable Rabbonim would not fault you if you opened up a Gemara on your own and started learning.