There are a couple of places in the book where the physical location of places is mentioned. The first is in the beginning when the street the Finches live on is described.
In chapter 1, Scout says that they live on the “main residential street in town” (p. 3), implying that there are not many streets with houses on them. There is a post office at the end of the street, and the mean old lady Mrs. Dubose lives two doors up the street from the Finches and “the Radley Place three doors to the south” (ch 1, p. 3), and Mr. Avery boards across the street from her (ch 6, p. 35). Ms. Maudie lives next door to the Finches.
The courthouse is in the town square, “wedged between Tyndal's Hardware Store and The Maycomb Tribune office” (ch 15, p. 106).
Starkly out of place in a town of square-faced stores and steep-roofed houses, the Maycomb jail was a miniature Gothic joke one cell wide and two cells high… (ch 15, p. 106).
That should be enough to get you started!