Your best bet is to order a replacement drive from Dell, as their machines tend to be specific in what they can run.
There should be a small hard drive symbol on the bottom of the laptop. There should be 2 or 4 screws nearby. After removing the screws and taking off the plastic panel, you should see the hard drive. Disconnect the connectors that connect the hard drive from the laptop and remove it.
Just do the opposite to put it back together. You'll have to reinstall the OS at first boot by booting from the CD-ROM drive.