Your password is something that you created so unless you wrote it down you will not be able to find your password for your secured internet connection. However, if you have an internet/phone company like Verizon then your password should be your phone number. I've never had internet through a cable company so I am not sure if the password would be on the back of the box. You can always call your internet service and ask for the password. I hope this helps!
Look in your routers documentation for security settings. You want to turn on something called WAP. You put a "key" on the router and the same "key" on your PC, then only equipment with the key value you set will be able to access your wireless connection.