Replace your existing router with a "wireless" one. You could simply plug a Wireless Access Point into one of the existing router's LAN ports, but you will almost certainly find you can buy a wireless router for less than a stand-alone W.A.P.
The devices connecting wirelessly then need to have a Wireless Network Adapter installed.
Sharing the internet wirelessly is really no different from what you are doing at the moment, except you have to establish the wireless connection between computer and router. This emulates a normal ethernet connection but over a wireless link.