Replacing an old dishwasher is usually just a matter of disconnecting the old machine, noting down all of the connections, then installing the new machine according to the manufacturer’s instructions. Most appliance stores offer free installation when purchasing a new machine. For detailed instructions on replacing an old dishwasher, check out this article from Popular Mechanics.
If it is a new dishwasher installation, you should be prepared for a more difficult task. Once you have framed in the opening using the manufacturer’s specifications from the instructions. You must run the power line to the dishwasher connection on the bottom, run the drain line to an air gap on the sink, and tap in a water line from the hot line for the sink to the dishwasher water hookup on the bottom. When in doubt, enlist the services of a professional. If you’re installing a new dishwasher, it’s a good idea to install a shut off under the sink at the same time.