3) change the router password (and document it!) save the settings (which will restart it)
4) you apparently have wifi conectivity for the systems, so you need only to
enable encryption and set the passphrase {use WPA2, or WPA, and avoid WEP if possible)
save the settings and disconnect the wired system
each system will need to connect the first time and you will need the passphrase to do so.
So enable the wireless security use a WPA Pre Shared Key in the security Mode and enter my own WPA Shared Key and the wireless device will ask for the Shared key I entered? Thats it and my network will be secured?