Upgrading from Blackberry Torch to an iPhone 4s?

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I got an iphone from my best friend for free (her whole family got the 5 and had their 4s's left over). I've currently got the bb torch 9800. I'm nervous for upgrading tomorrow (need to get a new sim card and plan for the iphone).
My family is strapped for money right now, so I have to keep my phone plan under $45. I'm locked into Telus canada, but I can easily change my phone and plan, I just need to stay with them until 2014.
I plan on getting a Voice plan that's got call id, voicemail, 200 minutes + free nights and weekends, unlimited pic, video and text messaging, as well as a $10 data add on. The data I get for $10 is 100MB. Is this enough to use for going on Facebook, Twitter, and googling things when needed? (does iMessage need data or does it work like texting?) Also for apps that may need data. I'll use my home's wifi and school's wifi when possible. How do I turn off the data to use wifi instead?
I can't use the iphone yet because theres no sim card in it, so i cant try it out or anything. tomorrow im going to buy a sim card, and switch my plan.

Is it a smart move to upgrade to the iphone 4s from a bb 9800? I do love my qwerty keyboard and BBM, but i dont know.