I want to dunk a basketball - I need to improve my jumping (and overall fitness) What are good exercises?

The primary muscles used when jumping are your quads and calf muscles. So, I would start by doing calf rasises and squats. You should see a difference pretty quickly.

calf raises and squats r good i do seated and standing calf raises and squats with a barbell an if it gets to easy add more weight also try doin squats on ur toes and trycalf raises without wieghts just put ur hands against a wall and tip toe repeatedly but keep it under conrtol dont jus hop up an down you feel the burn eventually
plus the wall sit helps put ur back against a wall an bend ur knees to a 90 degree angle this works your quads