to go into detail of AM's post. your motherboard is going to have 3 jacks, real wide. a wide flat wire's running from each them. then smaller one iirc would most likely be going to your A: drive, or floppy w/e. Another will be going to your CD-rom/DVD-rom(s), and the last will be your HDD. these wire comes in single and dual jacks which you can plug devices into (ie. cd/dvd/hhd drives). If your adding a drive, your naturally going to have to have a dual wire. before you plug it in, you should find the manual for the HDD itself or the computers, and see the configuration for SLAVE set for the HDD (each HDD is differant). there is also CABLE SELECT setting that may or may not do the same thing.
once you have your additional drive set to slave, place it where ever you want, hopefully a open drive bay and not ducked taped to the side like my old ghetto ass. plug the secondary drive into the middle of the wire's, jack. it should be labeled SLAVE anyway where you should plug it in. and the main drive gets the end of the cord. fire up your PC and install the proper drivers from the HDD's packaged cd / floppy or use the company CD that came with the entire package if you bought from best buy or some noise.