Hi Trevor
On a Mac, the Finder programme is the equivalent of Windows' Explorer.
When you launch Finder, on the left side of the window, under 'Locations' you'll see your iMac and if you've inserted one, your USB stick. They usually have a name which I obviously don't know here but I'm sure you'll recognise it. You can rename the stick by right-clicking on its name and taking the rename option.
If you left-click on the USB stick, the contents will be shown in the right window.
One other thing, unlike Windows, the Mac operating system, requires you to 'eject' any external drives before you remove them. You do that by clicking on the small triangle next to the disk name on the left-hand window. If you forget to eject properly, you'll get a message reminding you to eject in future.
It might be useful to have a look at one of these YouTube videos:
https://www.youtube.com/results?search_query=new+to+macCheers
Chris