Servers usually have training classes. Each parish and sometimes even each priest has preferences as to what the servers should do at different points in the Mass. The attire is also different at different parishes. Ours uses albs rather than cassock and surplice sets.
You would most likely either wear a cassock and surplice, or an alb, with amice and cincture if necessary.
In my experience, this is what an altar server does (it might be different depending on the parish or region):
Before Mass,
they ensure that the Sacramentary (in some places there are two, a large one and a small one), Lectionary, and other needed liturgical books are in their propers places.
they ensure the items needed for the Liturgy of the Eucharist (chalices, ciboria, cruets, etc.) are on the credence table or their proper spot.
they ensure the candles are lit (two are required, there might be more depending on where you are).
During Mass,
they walk in the procession, possibly carrying items like a crucifix, candles, etc.
they might bring the Sacramentary to the celebrant (might be the smaller one) for the Opening Prayer.
they actively participate by saying the people’s parts with the congregation.
they may carry candles in the procession to the ambo for the Gospel.
they sit and listen attentively to the homily.
they might bring the Prayers of the Faithful to the celebrant for him to say his parts, if he has parts in it.
they help the ministers prepare the altar for the Liturgy of the Eucharist (logistics vary widely from place to place).
they could possibly ring bells during the elevations (depends on the parish or priest).
if the Sign of Peace is used, they offer the Sign of Peace to the others in the sanctuary.
in some parishes, they hold the communion paten underneath people’s chin or hands during the distribution of communion.
they might pour the water for the purification of the vessels.
they would possibly bring the celebrant the Sacramentary for the Prayer after Communion.
they would walk in the recessional, possibly carrying items.
After Mass,
they might take the sacred vessels into the sacristy.
they would put out the candles.
they would take into the sacristy whatever needs to be taken in.
they unvest, and that’s it!
That was by no means a comprehensive list, nor is it necessarily accurate for your church. Priests have preferences, and some churches do things differently. Ask either a sacristan, priest, or a person in the know about such things at your parish.
You didn’t mention if this is for a Sunday Mass or a weekday Mass. I sometimes serve for the weekday Masses when the deacon is away. My normal work attire is black dress pants and a uniform shirt. I just substitute a long-sleeve white dress shirt for the uniform shirt, and I’ve been fine. There should be some training involved, and JGMendes’ list of responsibilities is excellent.
I serve at our college chapel. We have 1 alter server. Attire is essentially “whatever you happen to be wearing” (there’s been scheduling difficulties, so often people are plucked right before Mass to do parts they’ve been trained for). I know that at the local cathedral for weekday Masses, the server is usually in business attire or something more or less formal.
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