How do you become lesbian without your parents knowing?

Lesbian is an identity. You can identify as whatever you want without anyone ever finding out; it's a mental thing. Having a gay relationship without your parents finding out is more complicated, but it's fairly easy to make up excuses for where you're going and who she is, and it's easy to talk at length over the internet. It's better to be honest if you can, but if your parents are bigots there is little you can do.