What gateway? Is this one of Comcast's Xfinity Wireless Gateways? If so, what model? If not, where did you get it and who set it up?
If you don't know if the wireless network you see is yours, then turn the gateway off (pull its power cord) and look at your wifi networks again, does the network disappear? If so that's your network.
If you can't log into your gateway because you don't know the admin password, then try the default. On Xfinity residential gateways (usually at http://10.0.0.1), the default userid is "admin" and the default password is "password". If they don't work, then EG's suggestion is the best solution. Simply press and hold the Reset button for more than 15 seconds and the device will have its setting changed back to all the defaults and you should now be able to login and change things, including the wireless password.
Another option if you have the Xfinity gateway is call the gateway support folks at 800-363-2416 and have them change things. But make very sure you know what the new admin is and verify you can log into the gateway before you get off the phone with them. Save the info someplace safe so you don't forget again.
If you have some other gateway or router, the process is usually very similar, but the address and default passwords will likely be different. Consult your manual.
