If you aren’t Catholic, entering a Catholic church for the first time can be like stepping into another world, or a bygone era. They have their very unique traditions and ceremonies, and everyone seems to know what to do, except for you. This can be an embarrassing, and even an intimidating experience, as you see people staring at you because you aren’t "doing things right."
First, make sure that you are respectful. This should go without saying, but you need to make sure that you aren’t talking or whispering through the service, whether it is a memorial, a wedding, or mass. Don’t touch any of the artifacts, never walk up close to the alter, and don’t pop open a soda in the middle of mass!
Secondly, dress appropriately. Everyone has heard of ‘church clothes,’ and you should try to dress as smartly as possible when you are visiting a Catholic church. Every church is different, but it’s better to overdress than under-dress. Unlike some religions, shoes and jackets are fine, but it’s considered polite to remove your hat upon entering the church.
Third, pay attention when everyone is taking communion – the Eucharist and wine. Everyone will get up and file to the front, where the vicar or priest will give them a rice cracker and a sip of wine from a silver cup. This represents (respectively) the body and blood of Christ, and by eating and drinking it you are literally taking Christ inside yourself. Only Catholics can take communion, and they will be highly offended if you try to do it.
Fourth, watch what everyone else is going. In a Catholic service, there is going to be a lot of standing, sitting, praying and kneeling, and for those who are unaccustomed to it, it feels like an aerobic workout. Bring a knee-pad or cushion if you have bad knees or a bad back.
Fifth, when in doubt, keep silent. People at Catholic churches would much rather you remain silent than interrupt their service. You’ll notice everyone singing, so join in with the help of a hymn book, but otherwise remain silent and polite.