How do I use an iPhone 3G without cell service on it?

I just bought a used iPhone 3G and I just want to use it basically as an iPod Touch for my kid. I can't get it to connect to Wifi and it's not compatible with my older version of iTunes on my iMac running tiger. What do I need to do to get it to be used as just an iPod? Thanks for your help!

Just update iTunes and try to "hacktivate" it (this means it doesnt need a sim card). I don't know what to do about the wifi