I do not know how to decipher the code, please can anyone help?

If you mean the ceasar cipher then, as explained by Jason's friend, it's just shifting the letters by X number. So, in his example (AQ), everything is shifted by 16: A becomes Q, B becomes R, C becomes S, etc.

Your clue is that there's a single letter word in the message. In the English language, that means there's only two possibilities for that single letter word. Everything falls into place from there.