Grate or dice cucumber, onion, and garlic (depending on your preference). Mix everything together. Chill.
I like to grate my vegetables; this results in a bit of a different texture as the water and pulp from the grated vegetables mixes with the Yogurt. If you dice the vegetables instead the texture is a bit more yogurty, and you need to refrigerate it for a bit longer before eating. Make it both ways and see; it only takes ten minutes to throw together a batch.
A good yogurt with cream makes for a very nice base texture.