alphabet

There is no perfect sign for "alphabet." 

Here are some approaches:

1.  Fingerspell  A-B-C normally, and then do the letter "S" with a twist in the wrist which represents an "apostrophe 's.'" and would basically mean, "The ABC's"  (That twisting movement is similar to the movement you make when biting off a piece of beef jerky--a forward rolling twist. But the location is not near the mouth, it is in the normal fingerspelling location.)

2.  Fingerspell the letters A-B-C and then you do the sign for "fingerspell."

3.  If that doesn't do the trick, you can spell A-B-C and then drag your finger to the right a couple inches (as if drawing a line in the air) and then sign "Z."  That would mean "A through Z," which would be understood to mean, "the alphabet."