Summarize this article:

Your computer should have a thin, wide slot on its housing; this is where the SD card goes.  Make sure you insert the SD card angled side first and label side up. Many newer Macs don't have an SD card slot, so you'll need to purchase an SD-to-USB adapter in order to connect your SD card. It's a blue, face-shaped icon in the Dock. This menu item is on the left side of your Mac's menu bar, which is at the top of the screen. It's in the Go drop-down menu. It's in the middle of the Utilities page. The utilities on this page are usually arranged alphabetically. You'll see it in the window that's on the far-left side of the Disk Utility page. It's at the top of the Disk Utility window. This box is in the middle of the page. Doing so causes a drop-down menu to appear with the following format options:   Mac OS Extended (Journaled) - The default Mac format. Only works on Mac.  Mac OS Extended (Journaled, Encrypted) - Encrypted version of the default Mac format.  Mac OS Extended (Case-sensitive, Journaled) - Version of the default Mac format that treats files with the same name differently if there are case differences (e.g., "file.txt" and "File.txt").  Mac OS Extended (Case-sensitive, Journaled, Encrypted) - A combination of the three above formatting options for the Mac format.  MS-DOS (FAT) - Works on both Windows and Mac computers, but has file size limit of 4 gigabytes.  ExFAT (Recommended) - Works on both Windows and Mac computers. Has no storage limit. Doing this will set your selected format as the preference for your SD card. Doing so will cause your Mac to begin erasing and reformatting your SD card. Once this process is complete, your SD card will support your selected format.

Summary:
Insert the SD card into your computer. Open the Finder. Click Go. Click Utilities. Double-click Disk Utility. Click your SD card's name. Click the Erase tab. Click the box below the "Format" heading. Click a format you want to use. Click Erase, then click Erase when prompted.