a) What is the first precept of the Church?
The first precept of the Church is: to attend Mass on Sundays and other holy days of obligation and to refrain from work and activities which could impede the sanctification of those days.
We sin against the first precept of the Church by not attending Mass and not taking some time for rest, unless we are excused for a serious reason, or by working unnecessarily, on the days indicated.
b) Why should an Altar Server know and live the first precept of the Church?
An Altar Server should be a good example in observing the first precept, especially because he is called to serve in Mass. His spiritual life would be greatly strengthened if he goes to Mass daily.
The Altar Server should also try to bring along his family and his friends when he goes to Mass.