Find out what type of cable system is used to connect drives to the motherboard. There are two systems in common use: IDE drives (also known as PATA, or Parallel ATA), and SATA (Serial ATA) drives. PATA drives have wide, flat cables or thick cables as wide as your finger, while SATA drives have thin cables about the diameter of a pencil. You will need to know whether to buy an IDE or SATA drive, and you should be able to tell by looking at the cables.
Now that you have confirmed that there is space to install a new drive in your machine and you know what type of drive you need (PATA or SATA), you can buy a new drive.