Do you have reffered to CLR.
I think if you have studied this algorithm atleast one you would not be asking this question here. I’m not discouraging asking questions.
But I’m suggesting you a way which would be more effective.
BTW here is answer.
So, every letter is assigned $2-bit$ code and we have total $13$ letter message so we need total of $26\space bits$ to encode this message.