Adobe Photoshop is an industry-standard photo manipulation tool for graphics professionals, but you can also perform a lot of drawing and painting tasks with it, as well as create 3D compositions. In fact, Photoshop is so versatile and complex that some tasks which are straightforward in simpler programs like Microsoft Paint may be more difficult for a beginner in Photoshop. Drawing and manipulation on a pixel-by-pixel basis is a good example of this, because Photoshop is geared towards high-resolution photos and effects with many pixels. However, working with individual pixels in Photoshop is certainly possible, and can be useful when working with very small images.
Select the Zoom tool from the Tools panel, the long narrow vertical panel usually found on the left side of the workspace. If you cannot find the Tools panel, click the Window menu and make sure "Tools" has a check mark next to it. The Zoom tool is the bottommost tool available, and looks like a magnifying glass. You can also select it by pressing Z.
Click repeatedly on your image in the area you'd like to select pixels in until the cursor icon no longer has a "+" symbol in it, indicating that Photoshop cannot zoom in further. Alternately, you can double-click the zoom percentage in the bottom left corner of your image, type "3200," then press Enter to quickly zoom in as far as possible. You should now see each pixel clearly defined in the image window.
Select the Rectangular Marquee tool from the Tools panel, or press M. If the Elliptical Marquee tool is showing instead (it has a round symbol), right-click its symbol in the Tools panel and choose "Rectangular Marquee Tool."
Choose "Fixed Size" from the "Style" drop-down menu in the Options panel, usually found just beneath the Photoshop menus at the top of the window. If you can't find the Options panel, click the Window menu and make sure "Options" has a check mark next to it. Type "1 px" into both the "Width" and "Height" fields in the Options panel, indicating that only one pixel will be selected at a time. Make sure the "Feather" field reads "0 px."
Click a pixel in your image to select it. The "marching ants" effect will appear as normal to show which pixel is selected, and you can use any of the tools as normal to manipulate that pixel. Shift-click to select multiple pixels at once and manipulate them together.
Use the Pencil tool to quickly colour one pixel at a time without needing to select it.