Use a photo editing software like Adobe Photoshop or GIMP. These programs allow you to adjust the color balance and contrast to create a black and white image that still retains the original depth and detail. You can also use the Black and White adjustment layer in Photoshop to convert the image to grayscale.
Use an online photo editing tool like Canva or Pixelied. These tools offer a black and white filter that you can apply to your images with just a few clicks. They also allow you to adjust the contrast and brightness to ensure that the image still looks great in black and white.
Use a specialized black and white conversion tool like Silver Efex Pro or Nik Silver Efex. These tools are specifically designed to convert color images to black and white while preserving the original detail and depth. They offer a range of presets and adjustments that allow you to customize the conversion process to your liking.
Shoot in black and white mode. If you have a camera that allows you to shoot in black and white mode, this can be a great way to capture images that are already in black and white. This mode will convert the color image to black and white in-camera, so you won't need to do any post-processing to get a black and white image.
Regardless of which method you choose, it's important to pay attention to the contrast and brightness of the image to ensure that it still looks good in black and white. You may also want to experiment with different filters or adjustments to find the look that works best for your image.