Under cabinet lighting is a neat thing.
Here are some different examples of options to use.
One option is the Lighting Kits that Digi-Key sells.
For example part number 1647-1059-ND is a Cool White LED. This will come with all the parts needed to install them.
This comes with the Power Supply, screws, brackets,power cables and LED strips.
With these kits the main thing to decide is the LED color and number of strips needed.
The Datasheet for these will explain how to hook them up and any other information you need.
Another option would be using LED Strips. These can be Flexible or Non-Flexible strips.
The Main thing when using the LED strips is the Power Supply or LED driver. These can vary on which one to use. You will have to look at the Voltage and Current ratings for the strips along with the number of strips that will be used. This will help to figure the correct power needed for the LED Strips.