A collar and leash designed for a dog isn't going to work for walking your cat (and it can actually damage your cat's trachea). The best way to walk your cat is with a harness, specifically one made for her and one where the leash attaches to the back of the harness (rather than to the neck, as it typically would on a collar).
Choose a feline-friendly harness that fits snugly around her torso with an easy-to-fasten buckle or Velcro. A rule of thumb on fit: If you can slide two fingers between the harness and her body, the fit is good. With a good harness at hand, let her get used to the harness before she wears it on the town; reward her every step of the way when you put her in her harness, and let her get used to how it feels walking around safely indoors before attempting your first outdoor walk.
Some cats may crave the outdoors, but others may be nervous or easily frightened. Begin your outdoor walks in a quiet, safe location, and don't expect to do much walking your first few times out the door. And remember: Be gentle, be patient. With a lot of praise and plenty of treat rewards, your cat will soon be taking you for a walk around the neighborhood.