Photoshop allows several different ways of doing this.
The easy one of course is to go Image>Mode>Grayscale Unfortunately easiest isn't always best.
Another way is to go Image>Adjustments>Desaturate or Image>Adjustments>Hue/Saturation and move the saturation slider to -100
Then again, and this one is nice, go Image>Adjustments>Channel Mixer check the monochrome box. Then play with the RGB sliders, the equivalent of using filters in the camera. Try to keep the total RGB value at about 100. If you like the results from using infra-red film try putting the green slider to +200 adjust the other sliders to taste. Then add some gaussian noise Filter>Noise>Add noise to give the grainy effect associated with infra-red film
Or, and this is the one I use most frequently,
change the mode from RGB to Lab Color Image>Mode>Lab
Color
Then in the Channels palette select
the Lightness Channel by clicking on it. The image will now become
monochromatic. This can be printed quite happily. Drag Channel b to the
trash bin.Drag the Alpha 2 channel to the trash bin. Now go Image>Mode>Grayscale
IF you want to be really fancy try this!
Open up the layers palette.
On the bottom bar of the layers palette click the new
adjustment layer button and select hue/saturation
Reduce the saturation to -100 and click OK
Select the background layer by clicking on it.
Create a second hue/saturation layer by repeating step 2. This will appear
on the layers palette between the background layer and the first hue/saturation
layer. Click the colorise check box and click OK
With this second hue/saturation layer (the middle layer) selected, change
the layer blending mode (it says Normal at the top left of the layers palette)
to color.
With this (middle layer) hue/saturation layer selected bring up the hue/saturation
dialogue by double clicking the adjustment layer icon on the highlighted
layer.
Moving the three sliders will bring about subtle changes in the in the image.
Deselecting the colorize button allows different effects when the sliders
are moved.
The effects can be made even more subtle by reducing the opacity of the
layer using the opacity slider at the top right of the layers palette.
When you are happy and before sharpening and printing Save the file as a
Psd file which will preserve all the layers. Then go Layers>flatten
image and apply some unsharp mask.
Then do a Save As give the file a different
name.
Print it!
Get all that? Worth the effort wasn't it?