Linksys/Cisco routers default to 192.168.1.1 (as well as many other manufacturers). If it is one you purchased Linksys/Cisco routers have a default user/pass of admin/admin. If it was supplied by your ISP they may have changed it and that information should be listed on the router or documentation they provided when installing it.
I have worked at several ISP's and if you have not setup a wireless network on this device before, or it has been reset, as neorecon19 stated you WILL need to be wired to the router to change this configuration. I don't know how many times I have heard "But I don't want to use a wire, I am going to be wireless", sigh, the hell of tech support... Again, if this is a device provided by your ISP they may already have a SSID and Key setup on it, and again that should be labeled somewhere on it.
If you still have trouble post what your routers Manufacturer name and Model number are, I am sure we can give you more detailed information on where exactly in the configuration menus to go, though thankfully they are pretty easy to navigate these days.