Don't try just a collar and a leash because your cat will decide they want to check something out or something scares them. I would start with a small vest, usually in the dog section, that will fit your cat. That will give you more control over your cat without the risk of escape. Once they've gotten used to the new sounds and sites of being outside you can switch to a harness. After a while, when you know your cat knows not to walk in the street and what is acceptable behavior on a leash, you can switch to a collar. But don't start with a collar.
The first time I tried to walk my cat on a leash, I had just a collar on him. Some little boy, about three, decided to chase after us because it's his first time seeing a cat. His mom chased him down but by then, my cat had slipped his collar and was around the building and halfway up the tree. Lucky for me I was able to grab him and calm him down before taking him home. I felt really bad because the kid just wanted to cuddle him but my cat really likes his space. The next time I got him a simple harness and he slipped out of it faster than the collar and we weren't even out of the house yet. So take my experience, and just get you cat a light vest first until your cat is trained.
