May I know how to check who is/are using ur wireless internet network? And how to track what they are doing?
You can check who is using your wireless by connecting to your router (either by cable or usually also by wireless, logging in on (usually) 192.168.1.1 or 192.168.0.1) - the routers control panel should have a section with connected users MAC addresses and computer names.
Tracking what they're doing isn't so simple, the easiest way to do that really would be to get onto the computers and check history and cookies, if you're concerned about sites being visited a lot of router CPs will have the ability to block websites by URL from the router.