microSD cards are small, flash-based memory, and so can be formatted by any device that is capable of reading the card.
One common way is to use a computer to format the card. This requires an adapter or card-reader that enables your computer to interface with the card. There are many USB-based card-readers available that can read microSD cards directly, or with an adapter to enable them to fit into standard SD card slots.
In a Windows computer, a microSD card will appear under "My Computer" as a "removable device". Right-click the icon for the drive and choose "format". Click the "Start" button in the dialog that appears, and the card will be formatted.
On a Macintosh computer, a microSD card will appear on the desktop. Open the application named "Disk Utility" and choose the microSD card icon from the list on the left. Choose the "Erase" tab. Click the "Erase" button and the card will be formatted.