wireless security

Mac Book,airport express.

When i open the signal icon at top of screen ,shows apple network number,airport turned on or off,,below that i have names appear at times and am not sure if they are using internet thru my computer.

Those would be other wireless hotspots that your Mac can see, not other clients connected to your network.

Just to be sure, make certain your wireless router is set to use WPA security.

Thanks for the reply,will try and find what router is set at