holmes4 is right. In *most* cases, cable Internet users have a modem directly attached to a PC, either via USB or ethernet, and buying a router with wireless is the correct move. In some cases, you can get wireless access from your cable provider by asking for a combination cable modem/gateway with wireless access built-in (I have a few clients with Comcast who have wireless available from their modem/router.
I have not seen too many cable providers using special set top boxes for Internet. AT&T's U-Verse uses their own gateway, but the TVs still have their own set top boxes for IP TV and controlling DVR functions.
The OP should detail who his provider and modem brand/model is, but chances are a router is what he needs.