Since the rope is your lifeline in rock climbing, it is important to get a good one. Thankfully, you won't have to set up physics experiments in your backyard to test the properties of various brands of rope, since the Union Internationale des Associations D’Alpinisme, or UIAA, has already done that for you. Most rock climbers feel comfortable with any new UIAA-approved rope. Do not buy a used rope. Ropes don't last forever, and can’t endure certain activities or treatment. You don't want to depend for your life on a rope that may already have been used to pull a prop plane out onto a tarmac.
Within the category of UIAA-approved ropes, you will have a number of choices. Do get a rope that is at least 165 feet long, and get a dynamic rope (for most activities) that has enough stretch to minimize the "impact force" on your body when you fall. It is also a good idea to look for a rope that has been waterproofed with silicon or fluorochemicals, since these coatings can minimize UV damage, as well as abrasion and water damage to the rope. Read the package information carefully to make sure you understand the claims the manufacturers are making for the rope. For information about how specific ropes handle, you may want to check a current gear review in Climbing magazine or another outdoor sports magazine. Expect to pay between $150 and $225 for a climbing rope.