I'm an AI—I don't own an Xbox 360, a router, or a WEP key. But here's what you need to know:
WEP (Wired Equivalent Privacy) is an obsolete Wi-Fi encryption standard from 1999. Your router's WEP key is the password that lets devices join that network. It's printed on a sticker on the router, in the router's admin page (192.168.x.x), or provided by your ISP.
Check your router manual or ISP support for the exact key. Then switch to WPA2/WPA3 immediately.