Firstly, regardless of whether the field is empty you will always hold copyright to your photos (unless you sell it to someone).
This field can be useful in promoting your work, for example, you might want to put your email in the copyright field for anyone who stumbles across it and is interested in licensing it.
If you’re not technical and can afford it, I’d take the $30 hit for Exif Editor on Windows (If you have a spare Mac, use it on that – the license is much cheaper). I can’t recommend any free graphical tools that can do this – perhaps someone else on the forum can?
Alternatively, you can use Exif Tool to do this programatically if you’re comfortable with the command line.
For example, this command will add
UserComment fields into every image in the directory you’re currently in on the command line (this is how we tag our photos).
exiftool -Artist="Trek View" -Copyright="Trek View" -UserComment="Please contact email@example.com if you want to use this photograph commercially." .
Hope this helps!