Are you seeking to rediscover your friendship with Jesus? Haven't been to Mass in a while and wondering where to start? Start by getting to know Jesus again through prayer. It doesn't matter how long its been since you have been to Mass. You are always invited to encounter Jesus in the Mass.  [USCCB]

Jesus said to them, "I am the bread of life; whoever comes to me will never hunger, and whoever believes in me will never thirst. "  [John 6:35]
-USCCB

Then he took a cup, gave thanks, and said, “Take this and share it among yourselves; for I tell you [that] from this time on I shall not drink of the fruit of the vine until the kingdom of God comes.” Then he took the bread, said the blessing, broke it, and gave it to them, saying, “This is my body, which will be given for you; do this in memory of me.” And likewise the cup after they had eaten, saying, “This cup is the new covenant in my blood, which will be shed for you.  [Luke 22: 17 - 20]

The Mass is much more than an ordnary church service.  Its central element is the Eucharist, which is the re-presentation (to make present) of Christ's sacrifice on the cross. In the Eucharist, under the appearance of bread and wine, we encounter the real presence of Jesus Christ in his complete humanity and divinity. Through it we receive the gifts of God's grace and unity with God and with all Christians (the Mystical Body of Christ). We recommit ourselves to the new and eternal covenant.

Watch Bishop Robert Barron's sermon [YouTube] on the biblical roots and spiritual meaning of the Eucharist.

The word "Mass" comes from the Latin word, "Missa." At one time, the people were dismissed at the end of the Mass with the words "Ite, missa est," meaning "Go, you are sent."

The Latin "Missa" comes from the word "missio," the root of the English word "mission." The liturgy does not simply come to an end. Those assembled are sent forth to bring the fruits of the Eucharist and the Holy spirit into the world. [U.S. Conference of Catholic Bishops]

The website of the U.S. Conference of Catholic Bishops lists a couple of different ways to describe the parts of the Mass:

In the Roman Rite, the Mass is made up of two principal parts: the Liturgy of the Word and the Liturgy of the Eucharist. It begins with the Introductory Rites and ends with the Concluding Rites.

Another way of dividing a Mass is into its "ordinary" parts—those texts which, with some variations, are part of the Mass on a daily basis—and its "proper" parts—the texts of prayers and selection of Scripture readings proper to the specific feast or other occasion being observed. [https://www.usccb.org/prayer-and-worship/the-mass/order-of-mass]

Watch Franciscan priest Casey Cole's series of videos [YouTube] explaining each part of the Mass.

Non-Catholics who are attending a Mass for the first time may be anxious about when to stand, when to kneel, and so forth. The simplest and perhaps best way is to watch what others are doing and do the same. After you've been to Mass a few times, you'll find that you're learning the routines of the Mass on your own. 

The bottom line is that as a visitor, no one will criticize you for not flawlessly performomg these practices. It will be obvious that you are new to this, and everyone was new at some point. We are just glad that you are here.

The only practice that everyone must observe (visitors and members) is that to receive communion you must be a Catholic and be properly disposed to receive it. 

New Catholics and visitors often like to have something to read and refer to during the Mass so that they can follow along with what's being said and done. Several types of references are available, each with a different purpose and each with its own pros and cons.
 

Order of Mass "pew cards," pamphlets, and other portable printed formats

This type of resource contains the "ordinary parts" of the Mass as described in the section "Parts of the Mass." Two reliable examples are:

• Liturgical Press:  https://litpress.org/Products/3378/Order-of-Mass-Hymnal-Insert
• Magnificat: https://bookstore.magnificat.net/cards/pew-cards.html

The problem with these resources is that they don't include the "proper" parts of the Mass: the specific prayers, scripture readings and Psalm responses that change from day to day.
 

Missals and Missalettes

These two resources include both the ordinary and proper parts of the Mass for each day, including Sundays. Missals are typically in hardcover book form and missalettes are softcover booklets that cover a fixed part of the year. Missals or missalettes are typically provided by the parish. They should remain in the church.
 

Monthly Subscription Publications

This is probably the best option because these monthly publications belong to you. You can use them to follow along with the Mass, just like a missalette. You can also read them at home and make them a part of your prayer and scripture reading. They contain a lot of additional content such as prayers, profiles of saints, essays, scripture commentaries and more. Two good examples are Magnificat and Give Us This Day. They publish once a month. The subscription cost is generally around $50/year.