It sounds like your phone is locked to your old (Tesco) network. For a phone to work on giffgaff, it needs to be unlocked, or locked to O2. Phones locked to any other network won't work.
You either need to get the phone unlocked by Tesco - ask them on the phone, they may just give you a code depending on how long you were with them), or get it unlocked at a phone shop / stall.
As for the PAC code, wait until the phone is up and running with your new (temporary) number, them type your PAC code in here:https://giffgaff.com/profile/details
Where it says "Keep my number"