you'll need a combined modem/wireless router.
You'll also need a wireless adapter for your PC if you want to use it any distance away from the router, although if the two are fairly close together (as they often are) you can connect the PC to the router via an Ethernet cable. One will come with the router, and you can connect that to the ethernet port on the back of the PC.
You'll need to connect a computer to the router intially anyway, so you can configure it with your ISP login details. You can use either machine for this purpose, it doesn't matter which.
Once the router is set up you'll see it make the connection, and then you can use your new latop anywhere within range. Don't forget to check if your laptop is one of those that has a wireless switch - some do - and if that's the case, turn it on.