The Mass is said in several languages, and the basilica asks those who come to dress appropriately and to behave as at any Mass. Groups of more than 25 are asked to fill in the basilica's registration form before coming. Times and days can occasionally change when a special event is held inside, so look at the basilica's calendar of Masses in the week you travel. Extraordinary Masses on great occasions are a different matter: those are by invitation only, obtained through the notice the basilica publishes for each one.
The basilica does not say how early to arrive. Because the places are limited and cannot be reserved, pilgrims commonly advise being at the Nativity façade well before the doors open, especially in summer and on feast days; treat that as travelers' experience and allow a generous margin.
Good to know before you go: coming to Mass is not a visit. The basilica's worship page speaks only of the celebration, and sightseeing is by timed ticket, with Sunday visiting hours beginning at 10:30 a.m. If you also want to see the church at leisure, the museum or a tower, book a visitor ticket for later the same day. The crypt, where Antoni Gaudí is buried, is the seat of the parish, which keeps its own schedule of worship and publishes its Mass times on the parish website.