Here are some of the most popular lenses around:
This is one of Canon's least expensive lenses, which is why it is so popular. It's wide open, fast, f/1.8 aperture is handy for utilizing the selective focus technique because of the very shallow depth-of-field (DOF) the lens can produce. The wide aperture is also useful when you don't have a lot of light available. Sharpest focus is obtained by using intermediate apertures, f/4 to about f/11.
Care must be taken with shallow DOF though because the DOF can get very thin, and you have to remember that the plane of focus is always parallel to the plane of the image sensor. If you want both of someone's eyes in focus in a portrait,