I'm looking at getting an Iphone tomorrow, but am unsure of how the data plans work?

Any app that is downloaded via 3g, updates infromation (like Facebook feed updates), email downloads, internet surfing, downloading picture messages... all of that uses data.

However, you can turn your 3g/4g off and use strictly wifi when around wireless internet like at the McDonalds or at home. This will NOT work against you data package. If you find you are always around wifi, you can always get by with your data off, and just turn it on when you NEED to use it.

I've been survivng on a 200mb plan for months by keeping it off and using wifi. And by surviving, I mean the Max I've ever used was around 32 MB. In one month.

PS. You turn it off usually in your network settings.

Nobody can give you a great answer ti that except a Rogers employee. Data plans, and their availability, pricing, and features are always changing and vary greatly. However, 100MB of data is very little, and I strongly recommend you go with something higher.