Grab your favorite programming language, iterate over each sequence position and raise a counter for the specifc dipeptide that you encouter. Hint: dictionaries/hashes are quite handy in such scenarios. Hint2: If you provide more input in your question, the more specific a possible answer gets. I.e. what are your programming skills, what have you done so far to come to a solution (sounds like a homework to me..), what constitutes a dipeptide for you (overlapping/non-overlapping)?

Chris

To calculate the numbers of different dipeptides check this thread