Generally, if you miss a pill, take it as soon as you remember. If it is the next day, and you haven't taken yesterday's pill take both pills that day to catch up. You generally won't get pregnant from missing one pill. If, however, you miss one of the first four or 5 pills in a new pack, you do have a significant risk of pregnancy.
Catch up on your pill and use protection (condoms, abstinence) for two weeks. If you miss two pills, many times you will have break through bleeding (bleeding when you are not supposed to be on your period). You should probably catch up on one pill, but taking three pills in one day will make many people sick (Nausea).
If you miss two pills in a row, one of the pills in the first week, or if you take antibiotics while on the pill, use a backup method for two weeks.