Neuroscience and learning of a single neuron
No, you cannot teach a single neuron an alphabet.

In general, the way the brain learns and stores information is by adjusting the strength of connections between neurons. Therefore, there is not really any information in a given neuron, but in the pattern of how it's connected to other neurons.

For the example of the alphabet, there are a lot of different pieces of information to tie together: there will be different neurons that respond to the visual shape of a letter, the word that names the letter, the sounds that the letter represents in spoken language, the words that contain the letter, etc.

A general neuroscience textbook like Purves is a good place to start learning about how brains work. You might also learn about how artificial neural networks learn. They are definitely different from biological nervous systems, and the learning mechanisms vary, but they share some characteristics such as the information being represented in the strength of connections between units, usually called weights in an artificial network.