@Julielil
You need a new PM SIM first.
If you are porting your old number, keep the old SIM in the phone and wait for the text confirming you are porting over to PM.
90 minute window to confirm.
Also have your old account number handy.
On your PC, go to the activation page as per @RossN.
Then follow the steps by entering the SIM number and your info. This will include picking a new number rather than porting your old number.
