Although this is officially the first day of your pregnancy, you won't conceive until around two weeks from now. This is classified as "day one" because once you conceive your pregnancy will be dated from the first day of your menstrual period. It would be more logical to date a pregnancy from the day of ovulation or conception, but, like most women, you're unlikely to know the day on which you ovulate, let alone conceive. You are, however, far more likely to remember when your last period started, especially if you're hoping to get pregnant and are keeping a record of your menstrual cycle.
While dating a pregnancy in this way is a handy, if slightly baffling, convention, it does mean that your body is getting geared up for pregnancy from today. In around 280 days, or nine months' time, you could be holding your newborn baby in your arms. Good luck and enjoy the journey!