The best subjects for candid street photography will be those who know you're taking a photograph, and either don't care or are active participants. This might be because they are part of a general street scene, but if you're obviously looking at a particular person the easiest way to find out how they would feel about being photographed is to ask them. You might have a few minutes when they're self-conscious or playing for the camera, but give it time. If they say "no", accept that. The idea of candid photography doesn't mean you have to sneak up on people.

Trying to take candid photographs of people who haven't given you their permission only really works if you're after angry or suspicious looking subjects.

Some photographers advocate a long lens. This will introduce a feel of separation between you and your subject, which works if that's the point, but doesn't if it isn't, and streets will feel more claustrophobic. There's a lot of good advice on lenses and types of camera here : What is a good focal length for street photography?