What was the first month on the roman calendar?

The very first calendar established by Romulus the founder of Rome and its first king (the calendar of Romulus) only had ten months: March to December (Martius to Decembris). The month was called Decembris because it was the tenth month. The name is derived from decem, the Latin for ten.

The second king of Rome, Numa Pomplius, introduced a new calendar (the calendar of Numa) and created twelve months by adding Ianuarius and Ferbuarius (January and February). The year still started in March and December remained the tenth month.