The medium decides two things: which microbes are allowed to grow, and what the ones that do grow look like. Put organisms into the sample, choose a plate, and the model works out the rest. Then tap a colony and read it.
Streak plate
The loop drags the sample out in a shrinking pattern. Growth runs together where it started. By the last pass the cells are far enough apart that each one makes its own colony, which is how you get a pure culture.
Reading a colony: note its form, meaning circular, irregular or punctate; its margin, meaning entire, undulate or lobate; its elevation, meaning flat, raised or convex; then size, surface and color.