what exactly is git-tag looking for when you try to sign a tag?

Since I reached my first release of my software this morning, I though I would 
try and get a bit more formal with things and attempted to sign the tag

Although I had tried gpg several years ago, I didn't seem to have any keys in 
my keyring, so I just generated a new one with 

gpg --gen-key

You can see a key listed with you check for secret keys

with gpg -K

but then I tried to generate a tag with 

git-tag -s v1.0

and it complained that I don't have any secret key available.

What exactly is the process of making one available?
-- 
Alan Chandler
http://www.chandlerfamily.org.uk
Open Source. It's the difference between trust and antitrust.
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