If you want to take candid pictures of people on the street you want to make sure that the person you want to photograph is unaware of your intention. Most people who become aware of being photographed will change their behavior and that's what you want to avoid. You want to be able to capture a pure and natural expression in your subject.

The easiest way to do so is to use a long lens so you can position yourself far away from whoever you want to photograph. Because there is a big distance between the camera and your subjects the viewer can sense it and cannot connect as well to the candid expression of the person in the image.

Another good method is to use a camera with a flip-out screen and a medium or wide angle lens. You have to position yourself in a location where your not in the way of people passing you by. Angle your body in a different direction where your lens is pointing, flip-out the screen of your camera and pretend that you are checking your previously taken pictures. That gives you a very inconspicuous appearance and you are able to take pictures of people close to you without them noticing it.

Obviously you can always shoot from the hip. The easiest way to do so would be with a wide angle lens. Despite the wide field of view you still have to practice to get your framing right. The large depth of field will make it easier to get your subject in focus but harder to separate it from the background.