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.