Make sure your router is connected to a power source and the Internet.
Connect to the default wireless network that your router creates (e.g. "NETGEAR32"). If it is locked the default password should be printed on the bottom of your router.
Open your browser and navigate to the default router configuration page. For NETGEAR this is https://routerlogin.net. The default login is "admin" and password is "password" though you should consult your device or manual for more information.
Click on the "Advanced" tab on the top of the screen. Then click "Administration" and "Set Password." Your old password is "password" by default and you should set your new password to something secure. Click "Apply".
Click the "Basic" tab on top. On the left hand menu click on the "Guest Network" tab.