If you are not a computer tech or technically inclined this can be quite difficult to accomplish. First off you'll need the right tools in order to do this. There are special screwdrivers that you'll need on hand in order to take the device apart. Next, a voltmeter is needed in order to test the voltage of the computer to make sure the power supply isn't burned up or there isn't a problem with the motherboard. Understanding where to test is important to make sure you receive the correct reading. Once you identify the problem it will be necessary to go online and find the parts that you need. Order these parts and then remove the damaged part and install the new part to the computer.
I would start off with testing the battery or even the power supply transformer that you use to connect the cable to the wall outlet. Many times the transformer goes bad and this can easily be replaced. Replacing the battery would require you to open the device and swap this out. If you don't have experience then it is best to visit the Apple store and ask for their help.