The wavelength is the length of one complete wave. It can be measured, on a distance graph, from any point to the next similar point on the wave. Remember one complete wave includes a peak and a trough.
The amplitude of a wave is the height of the wave from the middle to a peak or trough. This is often called the maximum displacement of the wave.
The crest is the highest point on a wave. It is the maximum positive displacement of a wave.
The trough is the lowest point on a wave. It is the maximum negative displacement of a wave.