There are a number of ways that this can be done.For our example I will take this picture of a young lady wearing a red kilt in front of a blue background. I make no great claims for it's artistic merit, but it is used to illustrate the points I will make.
1. Hit the greyscale button. This will give you a monochrome
image of sorts. It may be a bit flat and not have the tonal
gradation you are looking for. An adjustment of levels may improve matters (below).
2. Adjust the saturation to zero. This will almost certainly be the worst method you could use.Again an adjustment of levels may improve matters (below).
3. Effects plug-ins are available which allow you to see the effect of mixing the results from the three colour channels and adjusting the
brightness and contrast at the same time. This gives much more
control of the finished image.