Hi Joe,
Mulberry58 has given good advice. Any crucial files should be stored elsewhere whilst you sort out your new external drive.
Your new external drive may be from a PC or formatted for PC.
You will have to make it readable for Apple.
Any data that is on it will be erased as part of this process.
It is pretty straightforward. You use an Application called 'Disk Utility' which is on your Mac system. Found in Finder - then select Go and then drop down to Utilities. Open it up and if your new external disc is connected, disc utility will see it. Select your external disc, select Erase. In the drop down menus give it a name and select MacOS extended (Journaled).
See how you go. Hope this helps Joe. Post again and I'll check back if I can be any more help.