It’ll likely be already formatted, either for Windows (using NTFS) or for maximum compatibility (using FAT32). Once you’ve bought a new external storage drive, connect it to your Mac. How to Format Your Drive With Disk Utility
Let’s look at the best file system formats to use for your Mac's external drive. With the Disk Utility app, you can easily format the drive, give it a label, or even partition it and create multiple volumes.
Once you have it, your first task is to pick a suitable file system for that drive depending on your intended usage. The easiest way to add storage to your Mac is to buy an external hard drive.
