To convert an image from black to white in Photoshop, there are several methods you can use. The simplest method is to convert the image to grayscale, which discards all the color information in the photo. To do this, go to Image > Mode > Grayscale and click Discard.
Another method is to use the Black & White adjustment layer. To do this, go to Layer > New Adjustment Layer > Black & White. Photoshop will apply a default grayscale conversion to your image, but you can fine-tune your results in the Properties panel.
You can also use the Levels adjustment layer to adjust the light and dark levels of the input and output channels. To do this, go to Layer > New Adjustment Layer > Levels.
Finally, you can use the Saturation slider in the Adjustment Layers palette to strip out all but the gray tones. To do this, access the Adjustment Layer menu at the bottom of your Layers Palette using the half white circle, then click on it. Make sure it is on top, then in the Properties panel change the Saturation slider to zero.