go to your local supermarket/gas station/convenience stop, and purchase a minutes card that is the same as your phone's provider (telus, bell, rogers, fido, etc). follow the instructions on the card. usually it involves calling a number and entering in numbers.
make sure you get your receipt as sometimes the information (pin number etc) you need is on there.
make sure you get your receipt as sometimes the information (pin number etc) you need is on there.