1) Buy a wireless router.
2) Hook your cable/dsl modem to the WAN interface of it and your PC(s) to that on any other port.
3) Configure your wireless settings.
4) Setup the PS3 for the wireless settings you just made.
Off you go...
It's a little more detailed than that, but there will be certain differences depending on the router you get. Search online for what will work best for you as all that information already exists.