I never use the paintbucket tool, but if you're going to use it, first choose the brown color as your foreground color. You can do that by choosing a color in the Swatches panel or in the Color panel, or by using the Eyedropper tool to copy the color from anywhere on your computer screen. Once you have the color selected, you can click the Paintbucket tool in the area that you want to fill with that color. Note that the Paintbucket tool has a tolerance setting, The higher the number in the Tolerance field, the larger the area that will be filled with color, depending on how much contrast there is in the surrounding pixels.
If you post a screen capture of the image and what you're trying to do, I may have other suggestions.