Using contraceptive pills is one of the most effective methods for avoiding an unwanted pregnancy, together with condom and IUD, if used properly. Proper use means taking one pill every day during the whole menstrual cycle starting from the first day of the last menstruation. When you finish a pack with 28 pills, you start a new pack the next day. If the pack contains 21 pills you wait 7 days until you start a new pack. During those 7 days you should get your period.
If you use the pills properly (as I described you above) you will be protected from pregnancy during the whole cycle.