For first-time access to a router admin panel, the 192.168.1.1 login walkthrough on the homepage is the right starting point. It covers finding your gateway IP, entering the right credentials, and what to do if the defaults do not work. The guides below assume you can already reach the admin panel.
Topical reference guides on credentials, IP addresses, and brand-specific quirks live in the router brand directory and the IP address directory. Networking concepts (NAT, RFC 1918, what a default gateway actually does) sit in the learn section.