Warning: I am a complete tech novice.
I have an iphone 4 that is about to bite the dust. I am on a family plan with AT&T and was told when my brother upgraded his iphone that when I decided to upgrade mine, I would lose my unlimited data plan that I've had for years since it "no longer exists". I now have the oppurtunity to buy an iphone 5C from a friend. How does this affect my plan, SIM card, etc.? I have only ever purchased phones from the AT&T Store.