I'd say if you are not sure what you are doing then replace the existing drive would be the easiest option. (sorry to disagree ratter)
Things to watch out for are...
Check the master/slave setting matches the drive you remove, this is set using a small jumper block on the back of the drive (see pic below) and you simply move the little block to go between the pins marked as master or slave as required.
pic (this drive is set to slave, if the black jumper was pulled off and put back on the right hand set of pins it would make it the master)
http://members.cox.net/yesshows/CD6.jpg
make sure you don't force the power connector in as if you push hard enough it will go on the wrong way.
The long connector on the ribbon cable must be fitted with the little red stripe that's on one side of the cable closest to the power connector.
Other than that nothing else to do, there is no software required to make the drive work as a CD/DVD reader or a CD writer although you will need to install the burning software that came with the drive(if any did) to enable you to burn DVDs