like Mazing said . by using a condom there is little chance of being pregnant, so definitely start the pill tomorrow.
please note that the pill is NOT a replacement for a condom, once you start the pill it takes a while for it to be fully effective at preventing pregnancy, but even then it is NO help against Sexually transmitted diseases, so eventhough you take the pill it is still recommended that you continue use a condom also. you are young and even if you are faithful to your partner you can't be so sure you are his first, last or only!