To express the concept of the
"ABC's," you spell the individual letters A, B, and C, and then you add the
FINGERSPELL sign.
ABC's:





You can also express the concept of "the ABC's" by spelling A-B-C and then
signing "through"-(indexing a horizontal list) and then "Z."
If you need to be even "more" clear, you could sign:
ABC through Z and then use your index fingers to indicate that you are
talking about the letters as a "set."