First we will open the image in photoshop, for the point of this demonstration I will use a bunch of coloured flowers and turn it from coloured into black and white.

From here, we will click on the Menu 'Image', Adjustments and lastly, Black and White.
A window will come up with different options on how you want it to be black and white. You can just click on OK, if you want it to be black and white.
Otherwise, you can change with the settings.
Clicking on 'Tint', will tint the image with whatever colour you choose. For example if you choose a brown tint it will be a white and brown image.