What do people do at a Christening? Is it a regular church ceremony with baptism involved, or different?

I am a babysitter of a 6 month old baby who is having her Christening this weekend. What should I wear? and what happens at a Christening? Does it only contain people the family invited, or does everyone in the church attend? I really know NOTHING about it other than the fact I need to bring a gift. Do you bring the gift in the church or what? If you know ANY detailed info on this..please let me know! Thank you!