how to erase a usb flash drive

Drive Partition and Format - El Capitan or Later

1. Open Disk Utility in the Utilities' folder.
2. After Disk Utility loads select the drive (out-dented entry with the mfgr.'s ID and size) from the side list.
3. Click on the Erase tab in the Disk Utility toolbar. A panel should drop down.
4. In the drop down panel set the partition scheme to GUID. Set the Format type to APFS (SSDs only) or Mac OS Extended (Journaled.)
5. Click on the Apply button and click on the Done button when it is activated.
6. Quit Disk Utility.

Should you need a thorough "wash" then do the following: How to replace El Capitan's missing Secure Empty Trash - Macworld.

When the format of the drive is the same as when you bought it, use the method that Kappy proposes.

When the format is different now than when you bought it:

Start Diskutility,

Select the long name,

Select the Erase tab,

Change the name in Untitled,

Select: Guid PartitionTable (Master Bootrecord only for very small drives like 8GB)

Select the Format: was probably MS(Dos) Fat or ExFat: choose ExFat when the flash drive is more than 8GB