To see who's connected to your router: look at Attached Devices.
To secure its wireless: go to Wireless Settings and set authentication to recommended WPA2-PSK(Wi-Fi Protected Access 2 with Pre-Shared Key) with a strong Network Key.
1999: Freeserve 48K Dial-Up => 2005: Wanadoo 1 Meg BB => 2007: Orange 2 Meg BB => 2008: Orange 8 Meg LLU BB => 2010: Orange 16 Meg LLU BB