If you weigh 150 pounds and jump at 120 turns per minute, you'll burn about 12 calories a minute. It's lower in impact and easier on the knees than running, since you should jump only a little off the ground. Wear shoes with good support. Choose the right rope. Check the rope's length.
Some ropes come with instructions for adjusting the length.
It is best to jump on the kind of springy wood floor found at a gym or health club, but a lawn or a mat works well, too. Carpets are fine, but a thick one may throw off your timing. Concrete is too hard and increases the risk of injury, but if your shoes are good enough you should be able to jump anywhere. If you've been relatively sedentary and are just starting to jump rope, start at about 70 turns a minute, which allows you to double-hop each jump. Start with just a minute or two of jumping to give your shins and calves time to adjust.