As a newbie to Macs I thought putting files into iCloud would save me HD space, but now I see that is not the case. I also have Dropbox and I was wondering if there is a way to transfer files from iCloud to Dropbox and use Dropbox's selective sync option to keep some of the files stored there from cluttering up my HD. MacBook Air Early 2015, 121 GB flash storage, OS X El Capitan, Version 10.11.5.
You could just drag the files from the iCloud folder to the DropBox folder.
Do them a bit at a time to ensure that you have enough disk space to complete the transfer.
If your available disk space is less than the size of the largest file on your iCloud Drive (i.e. you have few, very large files) - you could upload it on the DropBox web site to store it in Dropbox without requiring extra local storage right away.