You could just use the old 'desaturate' option, or 'convert to grayscale', but you're missing out on one of the coolest features of black and white photography. It used to be something you had to think about when shooting monochrome film when you were actually shooting. FILTERS! You could shoot with different colored filters to enhance contrast or reduce it; punch certain colors, or effectively delete them. You could also do this in the darkroom when printing, by exposing the paper through a filter. You can (and should) do all this still in Photoshop. There are several ways to do it, but I like my method because it is infinitely adjustable. Enjoy.
Open an image in Photoshop and make two separate "Hue/Saturation Adjustment Layers" by following going to: Layers - New Adjustment Layer - Hue/Saturation.
The top adjustment layer, which will be used for Saturation, set the blending mode to "Color" and set the saturation to its minimum. This will change the photo to black and white, but the layer underneath will affect its tones.
The fun begins in the second layer, which we'll use for hue. Double-click on the layer to bring up the hue/saturation sliders, and slide the hue back and forth to apply different colored 'filters' to the black and white image. You can then adjust the saturation to enhance the strength of that filter.
Here's a shot of an apple that I took. The apple is red, but using filters, I can change perception so it can look like a yellow or green apple (to my eyes anyway).
The original (duh.)
Green/Yellow filter.
Notice how the apple gets very dark, while contrast in the grass is actually decreased. You will increase the contrast of a particular color with a filter of its compliment. Opposite applies with a filter of the same color. Red filter is below.
Huge difference. You can also use adjust the hue and saturation on a finer scale by adjusting each individual color band rather than master. Have fun messing with it.