well you cannot tether if you dont have service so forget about that.
I am not sure if you can use the phone without 'activating' it through itunes unless you unlock it or jailbreak through something like pwnage tool. there are GREAT step by step instructions (click here >
Pwnage 2.x Guide 3G - ModMyi - Wiki )
Once you get the phone turned on and past the slide to unlock part and can see your home screen you will be ready to set up your wifi. Mac's have the ability to share the internet. You can do this by going into system prefrences>sharing> then check off internet sharing>set your personal options (ex if you want to set a password and name your network)
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**IF YOU ALREADY HAVE WIRELESS BEING BROADCASTED THROUGH A ROUTER OR OTHER DEVICE YOU CAN SKIP THIS**
Now go to the system icon on your iphone and click on wifi and choose the network (you may or may not have to enter a password depending on your personal settings)
YOUR IN!! HAPPY BROWSING! Plus if you used pwnage tool to jailbreak youll have cydia and installer and you can download some nice applications as well.. not to mention the other things you can do that apple doesnt want you to do because they dont profit off of it :-p I wont get into that though.. haha good luck hit thanks if I helped.. You can PM me for specifics