doing this is really easy.
What you'll need, if not already part of what you have now for your broadband, is a router - in your case a wireless router.
Hope I'm not doing this too simple for you, sorry if I am
)
Instead of plugging the cable from your broadband modem into the computer, it now goes into the wireless router. Instead of entering your user datails - username and password - on the computer, you enter these on the router. Routers have software installed on them that allows you to enter user details, Protect your wireless network and lots of other stuff.
Depending on how you wish to connect your iMAC, you'll definately want to connect it to the router via network cable for the initial setup. Sometimes wireless isn't activated by the manufacturer, insome cases an initla password for wireless is printed on the bottom of the router. But go with a cable, because if you change a wireless setting, you'll get kicked out of the system and then have to use a cable for the setup anyway. Afterwards it's your choice if to stay with a cable or also go wireless.
This means that now the router controls internet access for you. It will open the internet for the first incoming computer, and extend the access to other computers.
On the router you will activate wireless functionality and wireless encryption. When you do so the system prompts you to create a password or passphrase. Depending on the encryption option you select this will be converted into a 26 digit number as a password (called WEP), or will be left as a password (called WPA or WPA2 - I recommed WPA).
Once you are finished, you can let your computers search for wireless networks. Once they find your router (the name you will see displayed on this list of wireless networks is usually the name of the product by default - but can be renamed for your ease - this is called 'SSID'), and you select it and click on it to connect, the computers recognises that encryption is in place and asks for a password. Only when they match can the computer connect. You can usually select that the computer can always connects to this network by default, so anytime someone wants to go online, they just start the internet browser.
Let me know if you need anything else. Potentially copy me on private message cos I don't always read this part of the forum.
Cheers,
Simon