How to read the screen pixels?

Starting with your code and omitting error checking ...

// Create a BITMAPINFO specifying the format you want the pixels in.
// To keep this simple, we'll use 32-bits per pixel (the high byte isn't
// used).
BITMAPINFO bmi = {0};
bmi.bmiHeader.biSize = sizeof(bmi.bmiHeader);
bmi.bmiHeader.biWidth = nScreenWidth;
bmi.bmiHeader.biHeight = nScreenHeight;
bmi.bmiHeader.biPlanes = 1;
bmi.bmiHeader.biBitCount = 32;
bmi.bmiHeader.biCompression = BI_RGB;

// Allocate a buffer to receive the pixel data.
RGBQUAD *pPixels = new RGBQUAD[nScreenWidth * nScreenHeight];

// Call GetDIBits to copy the bits from the device dependent bitmap
// into the buffer allocated above, using the pixel format you
// chose in the BITMAPINFO.
::GetDIBits(hCaptureDC,
            hCaptureBitmap,
            0,  // starting scanline
            nScreenHeight,  // scanlines to copy
            pPixels,  // buffer for your copy of the pixels
            &bmi,  // format you want the data in
            DIB_RGB_COLORS);  // actual pixels, not palette references

// You can now access the raw pixel data in pPixels.  Note that they are
// stored from the bottom scanline to the top, so pPixels[0] is the lower
// left pixel, pPixels[1] is the next pixel to the right,
// pPixels[nScreenWidth] is the first pixel on the second row from the
// bottom, etc.

// Don't forget to free the pixel buffer.
delete [] pPixels;

Rereading your question, it sounds like we may have gotten off on a tangent with the screen capture. If you just want to check some pixels on the screen, you can use GetPixel.

HDC hdcScreen = ::GetDC(NULL);
COLORREF pixel = ::GetPixel(hdcScreen, x, y);
ReleaseDC(NULL, hdcScreen);
if (pixel != CLR_INVALID) {
  int red = GetRValue(pixel);
  int green = GetGValue(pixel);
  int blue = GetBValue(pixel);
  ...
} else {
  // Error, x and y were outside the clipping region.
}

If you're going to read a lot of pixels, then you're better off with a screen capture and then using GetDIBits. Calling GetPixel zillions of times will be slow.

You make a screenshot with BitBlt(). The size of the shot is set with the nWidth and nHeight arguments. The upper left corner is set with the nXSrc and nYSrc arguments.

You can use the code below to read the screen pixels:

HWND desktop = GetDesktopWindow();
HDC desktopHdc = GetDC(desktop);
COLORREF color = GetPixel(desktopHdc, x, y);