My staff has a very poor work performance .. I need to write a letter to the management head about this issue, how can I write the letter? please help me.

What is the real problem here?

Do you need more training to be a good manager?  Or do the staff need more training?  Does the team need more resources?   Did your predecessor hire the wrong people? 

Your letter to the management head should explain the problem and also the solution, including the approximate cost and timescale.