@Julez6 - are you a current customer or are you looking to activate a Public Mobile SIM card?
If you are a current customer and you never registered for a self serve account, you can do that here:
https://selfserve.publicmobile.ca/self-registration/
then, see @Yummy 's post for paying online through your self serve account./
Otherwise, there are voucher options available at various locations or instant tops, etc...see options for paying for your services here:
https://www.publicmobile.ca/en/ab/get-help/articles?category=paying-for-your-service
