When you put a new hard drive in your Mac—or connect an external one—you need to decide how to partition the drive and what file system to put on it. The easiest way to do that is with the Disk Utility in Applications/Utilities. Disk Utility shows a list of all available disks along with all the volumes (partitions) present on those disks. If you want to keep things simple, just select a disk, click on the “erase” tab and you can create a volume that uses the entire disk with a few mouse clicks.
However, if your needs are more complex, select the “partition” tab. There, you can divide the disk into several partitions. The advantage of having multiple partitions is that if one fails, the others may be unaffected.