what plan do you currently have? You'll need a new sim card, which is $10 for postpaid and $20 for prepaid. Any other charges that you may incur depend on the current plan and what features you're using.
The you will need to have a Telus or Koodo iphone or the iPhone will have to be unlocked for it to work with Koodo.
You should be to put your current plan on it. However if was an older BB plan note that data on an BB is different that data on an iPhone and will result in additional charges if you attempt to use data.