You best choice is to buy an external USB DVD-RW since it will burn faster then an internal one. If you still like to go with an internal one ebay is the best place to purchase one. The old drive should have a push in release to extract it on the bottom right side (a retaining screw may be installed under the laptop). The new one just slides in. You may need software depending on what you are burning and your OS.
you are going to have to by a new DVDRW/CDRW drive and swap it with the one already in your computer.
Is very easy to do, turn off your pc remove the power cord, open it, take out the screws if there are any or slide out the old dvd drive, unplug the internal power and IDE cables, install the new drive, attach the internal cables, close your computer and that is all.