The entrance to most Roman churches is from the portico. There may be a central door, or pairs of side doors. Many entrances, whether central or to the side, will allow you to step into an enclosed foyer that separates the light, sound, and air temperature from outside. Central entrance foyers have right and left side doors, to allow the flow of people to go in to the right, and exit from the left. If the central door is closed, use the right side door to enter and left side door to exit (though it will be to the right from inside). Stop inside for a few moments to allow your eyes to adjust from the bright, outside ambient light, to the darker interior light. Before you is the nave of the church.
Smaller churches may have only a nave, with side chapels directly adjoining the nave. These follow the Greek temple design that had only one interior space. Larger churches, following a Roman basilica design, will have a central nave, two rows of pillars that separate side aisle and chapels. I will write about those separately.
Now that your eyes are accommodated to the cool light of clearstory windows and stone work, find a pew to sit in before you fall over looking up at all those statues, frescos, mosaics, etc. Many of the columns lining Roman churches were repurposed from Roman buildings. These may topped with different orders of capitols: Doric (rounded pillows), Ionic (scrolls), or Corinthian (floral shapes). Between some columns may be niches with statues of Biblical characters and saints. Relief sculptures and frescos, again on Biblical themes, or church history, usually line the walls above the columns, and between windows.
To really kink your neck, scan the ceiling. In older style basilicas these are flat surfaces with corbelled sections. From the 16th century on, these became suspended plaster ceilings curved with the barrel vault, or cross groins between four columns. In the 17th century, the paintings extended the space of the nave into the sky with illusionistic columns, clouds, and foreshortened people either sitting on open-appearing roof structures, or clouds. Many churches with this style of trompe l’oeil will have a brass circle in the middle of the nave which indicates the exact spot where the illusion is most prominent.
To stretch your neck the other direction, spend some time looking at the floor. Many churches have simple to elaborate marble floors. Again, many marble building materials were repurposed from Roman buildings to make these floors. Facades were taken down, column sliced into thin sections, and geometric patterns arranged with different colors of marbles. You may notice many circular pieces of marble. These were probably sections of round columns. While many of the floors are primary decorative, some are placed for symbolic value. In St. Prassida, a slice of red marble (at the bottom of the photo, just the end of this stone is visible) represents the location of the well, where she collected and stored the blood of martyrs. Okay, maybe that is more than you want to think about while sitting in church. If so, you probably are not going to appreciate all those side chapels with frescos of how saints were martyred (crucifixion, roasted on grates, beheaded, skewered with arrows or on wheels with spikes…)