You need a "Wireless" Router/ADSL Modem to replace your existing wired one and a Wireless Network Adapter in the PC(s) that connect to it. You can then plug the router into the main socket in the kitchen and connect wirelessly from another room. Be careful where you locate a wireless router, because a fridge/freezer might well block the radio signal if it is between the router and the computer.
You could actually plug a "Wireless Access Point" into your existing router, which would effectively convert it to a wireless router, but "stand-alone" Wireless Access Points are expensive. It is usually no more expensive to buy a wireless router.