Username and Password to connect to the wi-fi or to access the network after connection? If it is a post sign in your browser should redirect on connection.
If it is a work or university network asking for username and password it is probably using WPA2 Enterprise so go to Wi-Fi, add a new network select the security type as WPA2 - Enterprise AES (more than likely EAP-TTLS on method and then the authentication method maybe Challenge Handshake) without checking that network though I am guessing.
The other thing, it might be the Proxy you are looking for, in that case edit the network and you will see the Proxy in different text color. Username and Password there.