how do I open a usb on my Mac, there is no icon on my desktop when I put it in

What kind of USB device is it - stick/external drive/camera, etc?

In Finder>Preferences>General tab you definitely ticked 'External Disks' or Hard Disks?

What happens if, also in Finder Prefs>Sidebar tab you select External Disks or Hard Disks - does it show in the sidebar?

If not, try restarting with the USB device plugged in - any luck?

If not, does it show up in Disk Utility in the Utilities folder?

If not, sounds like a duff USB device.

Click on Finder, Preferences, General and check the box to display the devices on the desktop.

I lost my icon for my usb on my desktop

Pleased to help and thanks for the feedback.