The difference between the lighter colours (which become highlights) and the darker colours (which often become like shadows) is called tonal contrast.
Simple shapes can be very effective in black and white photography. Look for shapes that have significant contrast and contain strong blacks. When colours are converted into black and white they become various shades of grey. The difference between the lighter colours (which become highlights) and the darker colours (which often become like shadows) is called tonal contrast. Shapes and subjects with lots of tonal contrast tend to stand out in a shot so a good starting point when approaching black and white photography is look for subjects with high tonal contrast. This is obviously the opposite to what we look for in colour photography, which is why aiming for black and white can sometimes bail you out when lighting is harsh, like during midday sunlight.