fist find your routers IP address.
type cmd in your run box, this will bring up a command prompt.
type ipconfig
this will list your IP as well as your gateway(router)
your routers IP will most likely be, but not limited too
192.168.0.1
192.168.2.1
192.168.1.254
10.0.0.138
once you know your gateway/router type http://IP_of router_here
it will ask you for your password. as suggested already it will be admin
admin
admin
password
blank
admin
admin
blank
once thru that stage check out your wireless settings, security is what you want..look up WEP, hell you can figure it out from here.
this is all assuming your modem router gateway has wireless built in, you may have an access point, but its much the same.
this is also assuming that this is your wireless network and not your neighbors.
NOTE : WEP has been broken for a longtime, do not use WEP as it will only keep the honest crooks out.use WPA with a password phrase that is not in the dictonary.