you will need to access your router to check both of these – computer-adviser.com/access-home-router.html then look for the wireless section and make sure security it enabled, if possible use WPA and then set a password, any computer then connecting wirelessly will need to enter this password.

As for seeing who is connected look under the general section of your routers admin console and you will be able to see all the users, the best way though is to implement your security and then reboot youer router, this will kill all network connections, then anyone connecting from now will need to enter the security credentials

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