You will need 8 output neurons to represent the 8 possible labels. Your labels would look something like:
A: [ 1,-1,-1,-1,-1,-1,-1,-1 ]
B: [-1, 1,-1,-1,-1,-1,-1,-1 ]
C: [-1,-1, 1,-1,-1,-1,-1,-1 ]
D: [-1,-1,-1, 1,-1,-1,-1,-1 ]
E: [-1,-1,-1,-1, 1,-1,-1,-1 ]
F: [-1,-1,-1,-1,-1, 1,-1,-1 ]
G: [-1,-1,-1,-1,-1,-1, 1,-1 ]
H: [-1,-1,-1,-1,-1,-1,-1, 1 ]
Then you know what letter is being recognized by which output neuron has the highest activation.
You will also need to normalize your inputs to the range [-1,1] for a neural network.