I would like to reiterate before expounding further about Mass that it is not necessary that you believe in anything before attendance. It is okay if you're just curious. Most likely someone will make a statement about welcoming visitors before getting started which ideally reminds everyone that they are supposed to be accommodating to people who don't know what to do.
There are aspects of Mass and Catholic culture about which -- in my opinion -- it is effortless to assimilate into compared to what you have to expect from other Christian communities.
At the same time, there is an inverse aspect which is quite difficult and intimidating. But the hardest part is the first step forward, after which you will always know that it was not as difficult as you imagined.
Here is my advice on how to prepare you.
Finding a Church to Attend
In my case, I just went on Google Maps and searched for Catholic church and discovered multiple options are in very close proximity. But how do you decide between them?
One of those convenient things I mentioned before is that unlike other denominations, Catholics are not coming to church based on who they have a preference for listening to but rather what they are there to do which is universally the same across any church of the Roman rite.
The thing to keep in mind though is that depending on where you live, it's possible that certain locations will be culturally oriented one way or another, which might not matter, but it could be a language barrier potentially.
The best indicator of whether this will be an issue is the parish website. If you go there and everything is in your language, you can take note of the Sunday Mass time and just pick the easiest one to get to. But if the website is a mix of languages you don't speak in the same sentences with photographs of flags you don't relate with, you might consider looking at the other choices to see if there is a better option.
Because Catholicism is so widespread across the world, a lot of people retain elements of their culture and some parishes reflect this or that more than others. But it's easy to clear up just by having a look at the online presence of the church.
Once you are attending weekly, it is convenient to go at the location most nearby. But if you're not committed yet, it's possible that finding the location with the most beauty will do something for you. If that's something you want to consider, go for it.
Dress with Proper Attire
The important thing to remember is that whatever you wear should be reverent. Clothes should not be drawing attention to your body. Some people dress formally but you'll notice many are more casual. For that matter some people are not wearing proper attire at all. But make a good first impression and do not be like them. Not having fancy clothes is perfectly fine. Just don't dress like you're going to the gym or the beach.
A good rule of thumb is maybe, "Would I wear this to a job interview?"
Arrive Early to Get a Good Seat
My church is packed every week. The people who get there at the last second are trying their hardest to find a place and squeezing through people. Make it easier for yourself and just be 15 minutes early.
Visitors and other newcomers generally sit towards the back. You might be tempted to get a seat at the far end of the pews, or against the wall, thinking that you can easily escape if you have to or that no one will notice you. And maybe that will help you feel more comfortable but I found that sitting in the center away from walls is preferable because you're not in the way of people who need to get up and walk around you. More on that later.
Silently Observe and Mimic
Earlier I said that there was an easy part: Everyone is at Mass to participate in a ritual that you don't know yet. Part of that makes it easy because you're not thinking about finding the right people or what you're going to say to them. It's more like going to an exercise class where you don't know the routine, and in a way, that's also hard.
But for someone like me, it's far easier when I can silently observe what everyone is doing and learn the routine than it is to go somewhere that people are going to walk up immediately and boom across the whole room: "Why, hello there! You're a new comer. My name is Obadiah! What church are you from!?" I would have never had the courage to go if it was like that. It's not.
After Sitting Down
You will notice that other people are genuflecting, bowing, and signing the cross before they take their seats. It's okay that you don't do this or even know about it. As long as you reverently take your seat, you're fine.
Standing, Sitting, Kneeling
Everyone else will know exactly when to stand, sit, and when to fold out the thing to kneel on. All you have to do is wait for them to all do it and then follow along with them. If for whatever reason you can't stand or kneel, that's okay. But you should do it if you can. It makes you feel involved.
Responsory Words
Listen to what the clergy says and listen when the people recite something back in response to them. At first you will just be soaking it all in. You won't even be sure what they said always. You'll mainly be surprised that everyone knows exactly what to say and how many things they recite. Over time it will be like the words of a song you've heard on the radio. It will just come to you.
You could look up and try to memorize every detail but it's only going to intimidate you and make you feel like you will never memorize everything. Truly, on your first time, you never would so just embrace not knowing anything. On a second visit you might have a look through the hymnal and try to see if you find the section on the "Order of Mass." Many people open it up and read off it once they know what they're looking for.
But the first experience should not be with your head in a book.
Sign of Peace
At some point toward the ending, the clergy will prompt everyone to acknowledge the people sitting near them. People will shake hands or put two fingers up in a peace sign and say, "Peace be with you." My first time I didn't know that you were supposed to say something specific and I just said, "Hey." It's fine if you don't do everything perfectly.
If you're socially anxious, keep in mind that it's not that bad because it's a very brief moment and everyone is trying to spread around the greeting to everyone in the vicinity, so no one focuses an excess of attention on you. It's formulaic. You don't even have to think of something clever to say.
Communion
There is going to come a point where all the practicing Catholics in good standing with the Church are going to line up row by row and exit the pew, walk to the front of the sanctuary, receive Christ, and return back to their seat.
This is incredibly awkward because you are not supposed to receive the communion (you're not a Catholic in good standing). Some parishes make it easy and allow us to go up with everyone else, and when it's our turn, we cross our arms over our hearts to communicate that we are not ready to receive. However I do not recommend that you do this on your first visit, as everything is done with body language and I've been told that this practice is not universal everywhere.
The best thing is to just remain seated as it is the easiest way to continue observing. The problem is that if you're sitting in a place where you're in the way, people will have to scoot past you. This isn't necessarily a problem and lots of people do it, but it's awkward depending on if you are in a place people need to be.
This is why I said earlier that if you're in the center, people can exit and enter the pews on either side of you.
That's pretty much it.
It's a very beautiful thing that everyone does together and it's rewarding to be a part of it. The more you learn, it will be very familiar -- and like any familiar thing very comforting.