Do you mean a secured network that isn’t your own? If so, well, that’s the very definition of a secure network - it has restricted access so that you have to be provided a password by an admin. So, in short, you need to find the admin and ask for a password. If you are at a coffee shop or someplace like that and you want to use their WiFi, just go up to the counter and ask. Most of them are happy to give you access, they just don't want it open for every Tom, Dick and Harry that isn't a customer using their WiFi.