Mass is the central act of worship in the life of a Catholic. Going to Mass is about spending time with God, but also receiving his grace - the inner strength to live the Christian life. The name 'Mass' comes from the final blessing said by the priest, in Latin ‘Ite missa es’ meaning “to send out” as Jesus Christ sent his disciples out to the world to take his teaching to them.
For more information on the structure and meaning of the mass: USCCB guide to the mass.A Walk Through The Mass
Catholics are asked to attend mass every Sunday.
If you aren't Catholic, or are but haven't been in a long time, you are still very welcome to attend mass. If you are unsure what to do, you can simply observe respectfully. During communion you may remain in your seat or come up for a blessing by crossing your arms across your chest.
For a detailed step-by-step, the document "A Walk Through The Mass" linked above may be helpful.