J.G.
If it is a Catholic baptism, your role for the most part is to stand there and smile. Many priests at some point will ask the Godmother to hold the child or to light a candle or something. I have worked at several churches (as an employee) and assisted the priest. Every priest I worked with gave excellent instructions. He usually talked to the couple and the Godparents just before the service to show that what would happen and to let them know what they needed to do. Also, the priest usually says something like, "I now ask the mother to give the child to the Godmother to hold while .... " You get the idea.
Do not be afraid to ask if something is unclear.
If it is a not a Catholic service, I would simply arrive early and ask the minister and/or other church representative if you he/she could talk to you about what is going to be expected.
Relax and enjoy. This is such an honor!