God bless for serving as what you truly are a Christian Witness for this child. The sponsor for first communion may be the baptismal godparents, or someone else the child or family chooses. There is no canonical (“official”) role or duty for the sponsor at this time during the Mass and reception of the sacrament, and there is no requirement for a sponsor for first communion. This person should be Catholic and have the other qualifications of a godparent (fully initiatiated, living in conformity with Catholic moral teaching etc) but as you have been honored to be ask, please do come you are most heartily welcome. The sponsor is supposed to help the child prepare for first communion, not only in the learning aspect, but in understanding and by example on how to receive worthily and the true meaning of Eucharist. Sounds like this parent, from what you say, needs all the help she can get, so the child is lucky to have you with her. The role of the baptismal godparent is to help the parents raise the child Catholic and see to his religious formation, and set a good example of the Christian life.
Wear what you normally would for church. If the parish has some other guideline they will send something home. If you are not Catholic you will not be receiving of course, nor should the parent if she is not in right standing with the Church, but that is her problem not yours. Every parish has its own customs, sometimes the children sit together and receive at the same time as a group, before the rest of the congregation. In other churches the child is brought up by his parents and/or godparents (they simply do not receive if they are not properly disposed). You may participate in Mass, the posture, responses etc. as you feel comfortable. No one will think anything of it if you prefer to sit for instance.
good gifts include a children’s bible (Catholic versions can be found on the web or a Catholic bookstore, which will also have gift sets, usually containing a prayerbook, rosary, perhaps a pin or memento). A statue of the child’s patron name saint is also a possibility. If you feel uncomfortable giving a specifically “Catholic” devotional gift, a picture frame would be a good choice. Often there are formal photos taken, or the priest will pose with the children after Mass for photos.
In some cultures there are monetary expectations on the godparents. For instance here with our dominant Hispanic culture, the godparent or padrino has a bigger role to play, is often the person who brings the child to the sacraments or religion classes, attends preparation meetings, and may be expected to pay for the childs outfit, party or whatever. That you would find out by discretely asking the parents. It can, in some countries, even be construed as constituting a quasi-legal guardianship if the parents cannot care for or raise the child.
again thank you for making this child’s reception of the Sacrament so important and special, you have been honored by this choice and have a chance to make a real difference for her.
a nominal donation, which can simply be placed in the collection basket, is common but in no way required, and is just that, a donation and not usually a large amount, and helps cover extra costs incurred by the parish.
