Just got a new flash drive but when I insert it in my MacBook Sir it lights up red for a couple of seconds, and then nothing shows up / in Finder or on desktop or anywhere. I tried rebooting my computer, removing thumb drive, re-inserting it, still n

You need to partition and format the drive before you can use it.

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.