If you ever feel lost, you can orient yourself by gauging where you stand in relation to the two main thoroughfares that intersect at the city's center: Broad (or 14th, Street, which runs north–south; and Market Street, which runs east–west. Where Broad and Market meet, neatly dividing the city center into four quadrants, you'll find massive City Hall, Philadelphia's center of gravity.
Within Center City, the numbered streets start on the eastern side, from the Delaware River beginning with Front Street (consider it "1st Street") all the way west to 25th Street on the banks of the Schuylkill (pronounced "SKOO-kull") River. In between Market to the north and Lombard to the south, most of the east–west streets have tree names (from north to south: Chestnut, Walnut, Locust, Spruce, Pine).
Center City has four roughly equal-size city squares, one in each quadrangle. In the northwest quadrangle there's Logan Square; in the southwest is Rittenhouse; in the northeast there's Franklin Square; and in the southeast is Washington Square. Running along the banks of the Delaware River is Columbus Boulevard/Delaware Avenue.