I am a non-Catholic (non-believer, actually) who, in an effort to foster some family unity, attends weekly mass with my Catholic wife. Although I am in the same pew every Sunday, as a non-Catholic, I suppose I am still sort of a guest. As a guest, I feel an obligation not to be a distraction to those who are there to worship. So, I try to just blend in and not break any big rules. (Such as presenting myself for communion or shouting “Amen! Preach it Brother!” during the homily.)
My suggestions: Stand when everyone else stands; sit when everyone else sits; kneel when everyone else kneels. If you’re not comfortable with it, don’t genuflect or cross yourself. (I don’t, and no one has ever seemed to notice.) Don’t go up for communion and don’t drink the holy water. Don’t ask where to hang your coat, because there are no coat racks. Otherwise, just go with the flow and enjoy what will probably be an interesting experience. And one last thing…be sure to stick around for the post-mass donuts.