Obviously no one should expect a baby to behave in any other way than full on baby-ness. But one thing we can start from that first visit is communicating how different and holy church is.
These wee ones understand more than we sometimes give them credit for. If you act as if church is a mystical place, they will pick up on that and respond in kind.
I find loud shushing rarely does anything but annoy other adults, but a soothing whisper directly in their ear of, “oh, we have to be quiet at Mass because Mass is holy,” is a good habit to get into from the beginning—for both of you! And it will continue to serve you well in the toddler years.
“Oh, we have to be quiet at Mass because Mass is holy!”
Sometimes I like to be a little dramatic about it, not in a way that expresses chastisement but rather with a little gasp that says, “isn’t this amazing??” Maybe channel some exaggerated fairy tale princess a la Amy Adams in Enchanted.
I also find “catch phrases” to be parenting gold in general. It keeps things simple and prevents me from turning into an adult from Charlie Brown. This is one you will be able to use for years to come.
And truly, for the first year and a half, nursing is probably the easiest way to quiet a fussy baby. I was always able to find a way to discreetly nurse in the pew, but sometimes I had persnickety babies that fared better nursing away from the action. It can take some trial and error to find a system that works for you and your infant. Babies are born whole people, and they can take some time to figure out.