Replying to Please Help - I am retarded when it comes to mobile phones

Okay. I can't seem to wrap my head around the whole unlocking/smartphone/SIM card thing so I need help. Here's the situation:

- Lost my blackberry, need a new one to put on my current plan. (BELL CANADA)
- Friend is going to give me her old blackberry for free (ROGERS CANADA)

how on earth do I go about getting this blackberry to work for me?

Please and thank you, huge amounts of karma to everyone who helps.

pce