First, make sure the party that gave it to you calls their carrier and tell them to release the PIN of that BlackBerry from their account.
Next, if you want to use it on a carrier on which it is not branded for, you will need to unlock it:
How do I unlock my BlackBerry? - BlackBerryFAQ.com
Then, you will need to establilsh service with your carrier and get a BlackBerry Data Plan from them.
You may wish to reload the OS fresh, etc., also.