(I'm assuming that you have a router if you are wireless)
You need to log onto the router.
You can either turn on access control ( if this is avilable on your router so that only wireless cards that you have specified can access the connection. (note this does not encrypt data) or you can turn on encryption. ( this is whre devices need to have a key entered before they are allowed to connect.
Yo should really have a look at your router manual and it should take you step by step on how to do this.